Preheat the oven to 450 degrees F.

Place the croissants cut-side up on a baking sheet. Bake for 5 to 8 minutes, until the croissants are toasty. Remove and let them cool slightly, then chop or tear them into pieces.

Heat the butter in a large skillet over medium-low heat. Stir in the onions and garlic and cook for 5 minutes. Stir in the mushrooms and cook for another 5, until the mushrooms are plump and juicy. Stir in the spinach until it wilts. Sprinkle everything with a big pinch of salt and pepper and stir.

Brush a 9x13 baking dish with melted butter. Combine the croissant pieces and the spinach/mushroom mixture together in the dish and toss well, until everything is combined.

In a large bowl, whisk together the eggs, milk, heavy cream and a big pinch of salt and pepper. Stir in 1 cup of the grated cheese. I also like to stir in a handful of fresh chopped herbs, like parsley and chives, but it’s up to you! Place the rest of the cheese in a bag stored in the fridge.

Pour the egg mixture over top of the croissant mixture in the baking dish. Press any of the croissants into the egg liquid, making sure they are covered and absorbed the eggs.

Cover the dish with plastic wrap and refrigerate 8 hours, or overnight.

Preheat the oven to 350 degrees F. Remove the dish 30 minutes before baking to take the chill off.

Remove the plastic wrap. Top with remaining grated cheese. Bake the dish uncovered for 45 minutes. Top with extra parmesan and a sprinkle of parsley before serving. Serve!