Preheat your oven to 350°F (175°C).
Grease a 9×13 inch baking dish.
Spread half of the cubed bread in the bottom of the dish.
Layer half of the cooked sausage or bacon over the bread.
Sprinkle half of each type of cheese over the sausage/bacon layer.
Repeat the layers with the remaining bread, sausage/bacon, and cheese.
In a mixing bowl, whisk together the eggs, milk, mustard, salt, pepper, and nutmeg (if using).
Pour the egg mixture evenly over the casserole.
If desired, sprinkle green onions and diced red bell pepper on top for added color and flavor.
Cover the casserole with aluminum foil and let it refrigerate overnight. This allows the flavors to meld and the bread to absorb the egg mixture.
In the morning, preheat the oven to 350°F (175°C).
Bake the casserole, covered, for 30 minutes. Then, uncover and bake for an additional 15-20 minutes or until the eggs are set and the top is golden brown.
Allow the casserole to cool for a few minutes before serving.
Enjoy your Christmas Breakfast Casserole with family and friends! Feel free to modify the recipe by adding ingredients like spinach, mushrooms, or different types of cheese to suit your taste.