Grease a 9x13-inch baking dish with butter. Make sure all sides are coated well so nothing sticks. This simple step helps the casserole release easily and makes serving much smoother later.

Place the bread cubes evenly in the greased baking dish. Spread them out so they cover the base completely. This creates a soft and fluffy layer once the casserole bakes.

Sprinkle chopped sausage or bacon over the bread cubes. Add chopped onions and diced bell peppers evenly. This step builds flavor and ensures every bite has a balanced mix of ingredients.

In a large bowl, whisk together eggs, milk, and cream until smooth. Add salt, pepper, and mustard powder. Mix well so everything combines into a rich and creamy base.

Add shredded cheddar and grated Parmesan cheese to the egg mixture. Stir gently so the cheese spreads evenly. This helps create a rich, melty texture throughout the casserole.

Pour the egg mixture evenly over the bread and toppings. Press down lightly so all the bread absorbs the liquid. This step ensures a soft texture after baking.

Cover the baking dish with plastic wrap or foil. Place it in the refrigerator overnight. This resting time allows the bread to soak up all the flavors and become soft.

Preheat the oven to 350°F. Remove the casserole from the fridge and let it sit briefly. Bake for 40 to 45 minutes until golden on top and set in the center.

Let the casserole cool for about 10 minutes before slicing. This helps it firm up and hold shape. Serve warm and enjoy a cozy, rich breakfast with family or guests.