Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with melted butter.

Lay six slices of crustless white bread in the prepared baking dish, pressing down gently to create a sturdy base.

Layer half of the deli ham and turkey over the bread, followed by half of the Swiss cheese.

Add the remaining deli ham and turkey, followed by the rest of the Swiss cheese, and top with the last six slices of bread.

In a large bowl, whisk together the eggs, milk, Dijon mustard, salt, black pepper, and cayenne pepper until smooth.

Pour the egg mixture evenly over the layered ingredients, pressing down gently to saturate the layers.

Drizzle any remaining melted butter on top, cover with aluminum foil, and let it rest for at least 30 minutes or refrigerate overnight.

Bake the casserole for 30 minutes covered, then remove the foil and bake for an additional 15-20 minutes until golden brown and crispy.

Allow it to cool for about 10 minutes before slicing into squares and serving warm, optionally dusted with powdered sugar.