Add the mushrooms to a large pan on high heat and sauté until there is no liquid.
Add the butter to the same pan along with the shallots and garlic sauté for a minute then add the spinach. Cook until most of the liquid is reduced and set aside.
Whisk together the eggs, cream and mustard.
In a large bowl tear up the croissants in 3” pieces and add the mushrooms and spinach and mix well. Add in the eggs and cream and cheese. Season with salt and pepper and nutmeg.
Pour everything into a buttered a 10x8’ baking dish, cover with foil and bake for 15 minutes. Then remove foil and bake for an additional 15 minutes.