Prepare the Bread: Cut the day-old bread into 1-inch cubes or lightly toast fresh bread cubes in a preheated oven at 350°F (175°C) for about 10 minutes.

Cook the Sausage: In a large skillet, cook the breakfast sausage over medium heat for 8-10 minutes until browned. Drain excess grease and cool slightly.

Add Vegetables: Stir in the chopped onion and cook for 5-7 minutes until softened. If using red bell pepper, add during the last few minutes.

Combine Ingredients: In a large mixing bowl, combine toasted bread cubes, cooked sausage, sautéed onion, red bell pepper, cheddar, and Swiss cheeses.

Make the Custard: Whisk together eggs, whole milk, dry mustard powder, salt, and black pepper until well blended.

Assemble the Casserole: Pour custard over the bread mixture, press down gently to submerge, transfer to a greased baking dish, cover with plastic wrap, and refrigerate overnight.

Bake: Preheat oven to 350°F (175°C). Remove and unwrap the dish, bake for 45-55 minutes until golden brown and slightly jiggly in the center.

Rest and Serve: Let the casserole rest for 10-15 minutes before slicing and serving.