Preheat oven to 350°F; grease a 9x13-inch baking dish.

Cut 1 loaf day-old bread into 1-inch cubes (about 8 cups). If fresh, toast on a baking sheet 5-7 minutes.

Cook 1 pound breakfast sausage in a large skillet over medium, crumbling, about 6-8 minutes until browned and cooked through to 160°F.

Add 1 diced bell pepper and ½ diced onion; cook 3-4 minutes until softened and remove from heat.

Whisk 10 eggs, 2 cups milk, 1 teaspoon dry mustard, 1 teaspoon salt, and ½ teaspoon black pepper in a large bowl.

Stir toasted bread, sausage-vegetables, and 2 cups shredded cheddar into eggs.

Pour into dish; press gently to submerge bread in egg mixture.

Cover with foil; bake 30 minutes.

Uncover; bake another 15-20 minutes until top is golden brown and center sets and reaches 165°F.

Rest 10 minutes before serving.