Croissant Breakfast Casserole with Spinach & Mushrooms
Sep 15, 2025
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This is the easiest Croissant Breakfast Casserole. It’s filled with slightly caramelized onions, mushrooms and spinach, plus plenty of melty Gruyere cheese.
If you’ve never bought the giant plastic clamshell of croissants at Costco, let this be your excuse. I’ve always been tempted by them, and this recipe is the perfect excuse to grab a package. Making a breakfast casserole with croissants feels a bit decadent, which makes this brunch casserole perfect for holiday celebrations or weekend gatherings when you need to feed a crowd.
Ingredients You’ll Need
Here’s everything you’ll need to make this croissant breakfast casserole. It requires a fairly short ingredient list that you can easily find.
- Croissants – Day-old croissants work best, since they soak up the egg mixture without getting soggy. Tear or cut them into bite-sized pieces for even layers.
- Olive oil – Used to sauté the onions, mushrooms and spinach. You can also swap in butter if you prefer a richer flavor.
- Yellow onion – Slightly caramelized to add natural sweetness and depth to the casserole.
- Mushrooms – Sliced and browned until golden, they bring an earthy, savory flavor that balances the richness of the croissants and cheese.
- Baby spinach – Adds a pop of color and a bit of freshness. It cooks down quickly, so you’ll need the full 5 ounces.
- Eggs – The base of the custard that holds everything together. Ten eggs give this casserole plenty of structure.
- Whole milk & heavy cream – The combo makes the egg mixture extra creamy without being too heavy.
- Salt & black pepper – Simple seasoning is all you need to bring out the flavors and richness from buttery croissants.
- Gruyere cheese – Melts beautifully with a nutty, slightly sharp flavor that pairs well with the vegetables.
How to Make Breakfast Casserole with Croissants
Cook the Vegetables – Start by caramelizing the onion until soft and lightly golden. Set aside, then cook the mushrooms until browned before stirring in the baby spinach. This mix adds savory flavor and freshness to the breakfast casserole.
Cut or Tear the Croissants – Use a sharp knife to cut large croissants into roughly 2-inch pieces, or simply tear them apart for a rustic look. The buttery layers are the heart of this croissant breakfast casserole with spinach and mushrooms.
Layer the Casserole – In a greased 9-inch by 13-inch casserole dish, add half of the croissant pieces. Layer on the cooked vegetables and half of the Gruyere cheese. Top with the remaining croissants, then pour the custard mixture (eggs whisked with milk, cream, salt and pepper) evenly over the top. Finish with the rest of the cheese.
Bake and Serve – Cover the Croissant Breakfast Casserole with foil and bake for 30 minutes at 350 degrees F. Remove the foil and bake an additional 10 minutes, until the top is golden and slightly crisp. Slice, serve and enjoy!
Frequently Asked Questions
Yes! While Gruyere gives this Croissant Breakfast Casserole a nutty, slightly sharp flavor, you can easily swap in Swiss cheese or a sharp white cheddar for a bolder bite.
This recipe is flexible. Try adding cooked red bell peppers, sautéed zucchini or even sun-dried tomatoes for extra flavor. Just be sure to cook out excess moisture before layering the vegetables into the dish, so your Breakfast Casserole with Croissants doesn’t turn soggy.
Yes, in part. You can cook the vegetables a day in advance, then layer it together just before baking. While this recipe is similar to some overnight egg bakes you might have tried, croissants will get soggy and lose their structure if you let the egg mixture soak for too long.
Let leftovers come to room temperature then store in the refrigerator for up to 3 days. To reheat, warm slices in a 350 degree F oven until heated through, or microwave for about 1 minute and 15 seconds. The oven method helps maintain the crispy, golden top.
Whether you’re serving this Croissant Breakfast Casserole for Christmas morning or Easter brunch, it’s sure to be a crowd favorite. Make a simple green salad to serve along side the casserole and cut portions down to feed even more guests.
Croissant Breakfast Casserole
Ingredients
- 6 croissants, cut or torn into pieces
- 2 tablespoons olive oil, divided
- 1 yellow onion, sliced
- 1 pint mushrooms, sliced
- 5 ounces baby spinach
- 10 eggs
- ½ cup whole milk
- ½ cup heavy cream
- 1 teaspoon salt
- ½ teaspoon black pepper
- 4 ounces gruyere cheese, shredded
Instructions
- Preheat oven to 350 degrees F.
- Heat 1 tablespoon olive oil in a large skillet over medium heat. Add onions and cook, stirring occasionally, until softened and just beginning to caramelize, about 15 minutes.
- Transfer onions to a plate. Add remaining 1 tablespoon olive oil to the skillet, then add mushrooms. Cook until browned and starting to crisp, about 8 minutes. Stir in spinach, a handful at a time, until wilted. Remove from heat.
- In a large bowl, whisk together eggs, milk, heavy cream, salt and pepper.
- Grease a 9-inch by 13-inch baking dish with cooking spray or butter. Layer in half of the croissant pieces. Top with onions, mushrooms and spinach, then sprinkle with half of the cheese.
- Add remaining croissant pieces. Pour egg mixture evenly over the top and sprinkle with remaining cheese.
- Cover with foil and bake for 30 minutes. Remove foil and bake for another 10 minutes, or until the top is golden and slightly crisp.
- Let rest for 5 minutes before slicing and serving.
Like this? Leave a comment below!
My family will love this!