One good recipe for an overnight breakfast casserole is the cheesy sausage breakfast casserole from CHOW.com. Assemble the casserole ahead of time, and store it in a refrigerator overnight before baking.
To make cheesy sausage breakfast casserole, heat 1 tablespoon of vegetable oil in a large frying pan, and add 1 pound of uncooked pork breakfast sausage. Cook the sausage until it's done, which is typically around 10 minutes, breaking it up into crumbles with a spoon or spatula as it fries. Remove the pan from the heat, and allow the sausage to cool for 10 minutes.
Use a whisk to blend 12 large eggs in a large bowl, and then add 2 1/2 cups of whole milk, 2 tablespoons of ground mustard, 1 teaspoon of kosher salt and 1/2 teaspoon of freshly ground black pepper. Combine these ingredients, and add the sausage, 5 1/2 cups of cubed French or sourdough baguette, 3/4 cup of shredded sharp cheddar cheese and 3/4 cup of Monterey jack cheese. Stir the ingredients again, and then spread the mixture evenly into a 13-inch by 9-inch baking dish. Cover the dish with plastic wrap or aluminum foil and refrigerate overnight.
When it is time to bake the casserole, allow it to sit at room temperature while preheating the oven to 350 degrees Fahrenheit. Uncover the casserole, and bake it at 45 to 55 minutes or until a toothpick inserted into the center comes out clean.