- Butter, for the dish
- 2 tbsp. extra-virgin olive oil
- 16 oz. sliced cremini mushrooms
- 12 mini croissants
- 3 c. baby spinach
- 5 oz. Gruyère cheese, coarsely grated (about 1 1/2 c.)
- 8 large eggs
- 2 c. whole milk
- Kosher salt and pepper
- Butter a 9″-x-13″ casserole dish. Heat oil in a large nonstick skillet over medium heat and cook the mushrooms, stirring occasionally, until lightly golden brown, 6 to 8 minutes.
- While the mushrooms are cooking, partially split the croissants, making sure not to cut all the way through. Stuff with spinach and all but 1/2 cup cheese and transfer to the prepared dish, cut sides facing up. Divide mushrooms among the stuffed croissants.
- In a bowl, beat eggs with milk and 1/2 teaspoon each salt and pepper, then pour over the croissants. Sprinkle remaining 1/2 cup cheese over the top. Cover with foil, then plastic wrap; refrigerate overnight (or at least 15 minutes).
- When ready to serve, heat oven to 375 degrees F, remove casserole from fridge and discard plastic wrap. Bake for 20 minutes, then remove foil and bake until eggs are puffed and just set, 25 to 30 minutes more.