Cut croissants open (as if making a sandwich) and arrange them in greased pan, open side facing up.
Fill each croissant with bacon and cheese.
In a large bowl, beat together eggs, mustard, salt and pepper and set aside.
Pour mixture over croissant sandwiches. Cover with aluminum and let sit in the fridge for at least 30 minutes to overnight.
Preheat oven to 350ºF about 15 minutes prior to baking.
Bake covered for 30 minutes. Then remove aluminum and bake another 5 to 10 minutes or until the top has gotten crispy and browned a little. Let cool for at least 5 minutes before serving.