When you need a breakfast dish that’s elegant enough for a holiday brunch but comforting enough for a lazy weekend morning, this Mushroom and Gruyère Croissant Breakfast Casserole checks every box. This luxurious make-ahead dish combines buttery croissants torn into chunks with sautéed mushrooms, nutty Gruyère cheese, fresh herbs, and a rich custard that soaks into every flaky layer overnight. The result is something that tastes like a sophisticated cross between bread pudding and a savory strata, with crispy golden edges and a creamy, custardy interior that’s absolutely irresistible. The earthy mushrooms and sweet caramelized onions balance the richness of the eggs and cream, while the Gruyère adds that distinctive nutty, slightly sweet flavor that makes Swiss cheese so beloved. This casserole is perfect for Christmas morning when you want something special waiting in the oven, for Easter brunch when you’re feeding a crowd, or for any time you want to impress overnight guests without doing any work in the morning. Simply assemble it the night before, refrigerate, and bake it fresh while everyone gathers around the table.
Serving Quantity: Serves 8-10 people
Cooking Time:
- Prep time: 25 minutes
- Chilling time: 4 hours or overnight
- Baking time: 45 minutes
- Resting time: 10 minutes
- Total time: 5 hours 20 minutes (including chilling)
Nutrition Information (per serving):
- Calories: 485
- Total Fat: 32g
- Saturated Fat: 17g
- Cholesterol: 265mg
- Sodium: 620mg
- Total Carbohydrates: 32g
- Fiber: 2g
- Sugars: 6g
- Protein: 18g
- Calcium: 30% of daily value
- Iron: 12% of daily value
- Vitamin A: 25% of daily value
- Vitamin D: 15% of daily value
Ingredients
- 6 large butter croissants (preferably day-old)
- 1 pound of mixed mushrooms (cremini, shiitake, oyster)
- 2 cups of shredded Gruyère cheese
- 1 large onion thinly sliced
- 3 cloves of garlic minced
- 8 large eggs
- 2 and a half cups of whole milk
- 1 cup of heavy cream
- 3 tablespoons of butter
- 2 tablespoons of olive oil
- 2 tablespoons of fresh thyme leaves
- 1 tablespoon of fresh sage chopped
- 1 teaspoon of Dijon mustard
- 1 teaspoon of salt
- Half teaspoon of black pepper
- Quarter teaspoon of nutmeg
- 2 tablespoons of fresh parsley chopped for garnish
- Cooking spray for the baking dish
Choosing and Preparing Croissants
The croissants are the foundation of this casserole, so quality matters. Use real butter croissants from a bakery if possible, not the grocery store tubes. Day-old or even two-day-old croissants are actually ideal because they’re slightly stale and absorb the custard better without getting too soggy. Fresh croissants are so moist and buttery that they can turn mushy. If you only have fresh croissants, tear them into pieces and spread them on a baking sheet to dry out at room temperature for a few hours, or toast them lightly in a 300 degree oven for about 10 minutes. Tear each croissant into roughly 2-inch chunks. You want rustic, irregular pieces, not neat cubes. The torn edges create more surface area for soaking up custard.
Preparing the Mushrooms
Mixed mushrooms create the most complex, interesting flavor, but you can use all one type if that’s what you have. Clean the mushrooms by wiping them with a damp paper towel or soft brush. Don’t soak them in water because mushrooms are like sponges and will become waterlogged. Trim any tough stem ends. Slice the mushrooms about a quarter inch thick. Smaller mushrooms like cremini can be quartered. Larger ones should be sliced. The mushrooms will shrink significantly as they cook and release their moisture, so don’t slice them too thin or they’ll disappear. If using shiitakes, remove and discard the tough stems and slice only the caps.
Caramelizing the Onions
Heat 2 tablespoons of butter and the olive oil in a large skillet over medium heat. Add the thinly sliced onion and a pinch of salt. Cook, stirring occasionally, for about 15 to 20 minutes. The onions will slowly soften, turn translucent, then begin to turn golden and caramelize. Lower the heat if they’re browning too quickly. You want deep golden-brown onions that are sweet and jammy, not burned. This slow caramelization brings out the natural sugars and creates incredible depth of flavor. If you’re short on time, you can cook them on medium-high heat for about 10 minutes for softened but not fully caramelized onions. Transfer the cooked onions to a bowl.
Sautéing the Mushrooms
In the same skillet, add the remaining tablespoon of butter. Increase the heat to medium-high. Add the mushrooms in a single layer if possible. Don’t crowd them or they’ll steam instead of brown. You might need to work in batches. Let the mushrooms cook without stirring for about 3 to 4 minutes so they develop a nice golden-brown color. Stir and cook for another 3 to 4 minutes until they’re golden brown and their moisture has evaporated. The pan should look relatively dry, not watery. Add the minced garlic, fresh thyme, and sage. Cook for another minute, stirring constantly, until fragrant. Season with salt and pepper. Remove from heat and let cool slightly.
Assembling the Casserole
Spray a 9 by 13 inch baking dish generously with cooking spray or butter it well. Spread half of the torn croissant pieces in an even layer across the bottom of the dish. Scatter half of the caramelized onions, half of the mushroom mixture, and half of the shredded Gruyère over the croissants. Layer the remaining croissant pieces on top. Add the remaining onions, mushrooms, and cheese, distributing everything evenly. You want layers so every bite has all the components. Press down gently on the layers to compact them slightly. This helps the custard penetrate better.
Making the Custard
In a large mixing bowl, whisk together the eggs until well beaten. Add the milk, heavy cream, Dijon mustard, salt, pepper, and nutmeg. Whisk vigorously until completely combined and slightly frothy. The Dijon adds a subtle tang and helps emulsify the custard. The nutmeg provides warmth and a hint of sweetness that enhances the Gruyère. Taste the custard and adjust seasoning if needed. It should taste well-seasoned on its own because it needs to season all those croissants. Pour the custard mixture evenly over the layered croissants, making sure to get it into all the corners and gaps. Use a spatula to gently press down on the croissants, submerging them into the custard so they soak it up.
The Essential Chill
Cover the baking dish tightly with plastic wrap or aluminum foil. Refrigerate for at least 4 hours, but overnight is ideal. This resting time is crucial. It allows the croissants to fully absorb the custard, transforming from dry bread into a cohesive, custard-soaked base. Without this soaking time, you’ll have dry bread floating in liquid rather than a unified casserole. Press down on the casserole once or twice during the chilling time to help the custard distribute evenly. The top layer of croissants will be above the liquid at first, but as they soak, they’ll soften and sink into the custard.
Baking to Golden Perfection
Remove the casserole from the refrigerator about 30 minutes before baking to take the chill off. This helps it bake more evenly. Preheat your oven to 350 degrees. Remove the cover from the casserole. The surface should look wet but the croissants should be completely soaked through. Bake uncovered for 40 to 50 minutes. The casserole is done when the top is golden brown and crispy, the edges are bubbling, and a knife inserted in the center comes out clean. The internal temperature should reach 165 degrees. If the top is browning too quickly before the center is set, tent it loosely with aluminum foil. For an extra golden, crispy top, you can broil it for the last 2 minutes, watching carefully.
Resting and Serving
Remove the casserole from the oven and let it rest for 10 minutes before serving. This allows the custard to set completely so it doesn’t run when you cut into it. The casserole will be very hot and loose right out of the oven but will firm up as it cools slightly. Use a large serving spoon or spatula to cut and serve generous portions. The casserole should hold together but still be creamy inside with crispy edges. Garnish with freshly chopped parsley for a pop of color. Serve hot with your favorite breakfast sides.
What to Serve Alongside
This rich casserole pairs well with lighter sides that provide contrast. A simple green salad with lemon vinaigrette cuts through the richness. Fresh fruit salad adds sweetness and freshness. Crispy bacon or breakfast sausage provides salty, meaty contrast. Roasted tomatoes add acidity and color. Simple roasted asparagus brings elegance and vegetables. Smoked salmon on the side makes it feel extra special. Mimosas, Bloody Marys, or fresh orange juice are perfect beverages. Keep sides simple because this casserole is the star.
Storage and Reheating
Store leftovers covered in the refrigerator for up to 3 days. The casserole reheats beautifully. Reheat individual portions in the microwave for about 90 seconds, or reheat the whole casserole covered with foil in a 325 degree oven for about 20 minutes until heated through. The texture won’t be quite as perfect as fresh from the oven, but it’s still delicious. You can freeze the baked casserole for up to 2 months. Wrap portions tightly in plastic wrap and then foil. Thaw overnight in the refrigerator and reheat as directed. The texture changes slightly when frozen but it’s still quite good.
Make-Ahead Strategy
The beauty of this casserole is that all the work is done ahead. Assemble the entire casserole up to 24 hours before baking. Cover and refrigerate. In the morning, just pop it in the oven. You can prep components even further ahead. Caramelize the onions and sauté the mushrooms up to 2 days ahead and store them covered in the refrigerator. Shred the cheese ahead and store in a ziplock bag. Tear the croissants the night before. On assembly day, everything comes together quickly. You can also assemble and freeze the unbaked casserole for up to 1 month. Thaw overnight in the refrigerator before baking.
Variations and Substitutions
Use different cheeses like sharp cheddar, fontina, or Swiss if you can’t find Gruyère. A combination of cheeses adds complexity. Add cooked bacon, ham, or sausage for a meatier version. Include sautéed spinach or kale for more greens. Use different herbs like rosemary, tarragon, or chives. Add sun-dried tomatoes for sweet-tart flavor. Include roasted red peppers for color and sweetness. Use challah, brioche, or French bread instead of croissants. The texture will be different but still delicious. Add caramelized leeks instead of onions for a more delicate flavor.
Dietary Modifications
For a lighter version, use half-and-half instead of heavy cream and reduce the cheese to one and a half cups. Use whole wheat croissants or bread for more fiber. For vegetarian, this recipe is already vegetarian. Make it gluten-free by using gluten-free croissants or bread. The rest of the recipe is naturally gluten-free. It’s hard to make this dairy-free because the eggs, cream, and cheese are fundamental, but you could experiment with non-dairy milk and cheese alternatives. For lower cholesterol, use egg substitute instead of some of the whole eggs.
Troubleshooting Common Issues
If your casserole is soggy in the middle, it didn’t soak long enough or wasn’t baked long enough. Make sure to refrigerate at least 4 hours and bake until the center reaches 165 degrees. If it’s dry, you didn’t use enough custard or the croissants were too stale. Make sure every piece is submerged during soaking. If the top burns before the center cooks, your oven is too hot or you need to tent with foil. If it doesn’t hold together when serving, it didn’t rest long enough after baking. Give it the full 10 minutes. If the flavor is bland, you didn’t season the custard enough. Remember it needs to season all that bread.
Tips for Success
Day-old croissants are ideal. Fresh ones can become too mushy. Tear the croissants by hand for irregular pieces that absorb custard better than neat cubes. Don’t skip caramelizing the onions. This step adds essential sweetness and depth. Brown the mushrooms properly. They should be golden, not gray and watery. Real Gruyère cheese makes a difference. It has a distinctive nutty, sweet flavor that elevates the dish. Refrigerate the assembled casserole at least 4 hours. Overnight is even better. Press down on the croissants periodically during soaking to help them absorb custard evenly. Let it come to room temperature before baking for even cooking. Don’t overbake. The center should reach 165 degrees but shouldn’t be dry. Let it rest after baking so the custard sets and it’s easier to serve. Fresh herbs are important for bright flavor. Dried herbs don’t provide the same fresh taste. Season the custard well. It needs to flavor all those croissants. Use full-fat dairy for the richest, creamiest result. Low-fat versions won’t be as luxurious. A 9 by 13 inch pan is the right size. Smaller pans will be too deep and won’t cook evenly. This casserole is rich, so portions don’t need to be huge. It’s very filling. The edges get crispy and delicious. Don’t cover them up. They’re the best part. Serve it hot but not scalding. Let it cool just enough to eat safely. Fresh parsley garnish adds color and fresh flavor. Don’t skip this simple finishing touch. Make this for special occasions. It’s too decadent for everyday but perfect for celebrations. Prepare to receive compliments. This casserole always impresses.
