Put the croissants on a baking sheet and cover with plastic wrap or a damp cloth. Let stand at room temperature for 8-10 hours, or until they have doubled in size. Preheat the oven to 400 ° F (or 350 ° F convection). Brush the croissants with the egg and cook for 16 to 20 minutes or until they are golden brown. Cook within one week of receipt.