Cut 6 day-old croissants into 1-inch cubes. Butter a 9x13 inch baking dish and spread half of the croissant cubes in an even layer.
6 croissants, 2 tbsp butter
Scatter 2 cups diced ham over croissants, then sprinkle 1 cup shredded cheese on top. Add remaining croissant cubes, then top with remaining 1 cup cheese.
2 cups ham, 2 cups Swiss cheese, 6 croissants
In a large bowl, whisk together 6 eggs, 1½ cups milk, Dijon mustard, salt, and black pepper until smooth. Optionally, sauté diced onion in butter for 3 minutes and stir into the custard.
6 large eggs, 1.5 cups whole milk, 1 tsp Dijon mustard, 0.5 tsp salt, 0.25 tsp black pepper, 1 small onion, 2 tbsp butter
Pour custard evenly over layered croissants. Let sit for 10-15 minutes to allow croissants to absorb the custard. Press down gently with a spatula if dry spots remain. Dot 2 tablespoons butter on top.
6 large eggs, 1.5 cups whole milk, 1 tsp Dijon mustard, 0.5 tsp salt, 0.25 tsp black pepper, 2 tbsp butter
Bake at 350°F for 40-45 minutes until custard reaches 160°F internal temperature and the top is golden brown. Let rest 5-10 minutes before slicing and serving.
