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

Cut 1 loaf of day-old bread into 1-inch cubes (about 8 cups). Spread on a baking sheet and toast in the oven for 5-7 minutes if the bread is fresh.

In a large skillet, cook 1 pound of breakfast sausage over medium heat, breaking it into crumbles, until browned and cooked through (about 6-8 minutes). Ground pork must reach 160°F.

Add 1 diced bell pepper and ½ diced onion to the sausage and cook for 3-4 minutes until softened. Remove from heat.

In a large bowl, whisk together 10 eggs, 2 cups of milk, 1 teaspoon of dry mustard, 1 teaspoon of salt, and ½ teaspoon of black pepper.

Add the toasted bread cubes, sausage-vegetable mixture, and 2 cups of shredded cheddar cheese to the egg mixture. Stir to combine.

Pour everything into the prepared baking dish and press down gently to ensure the bread is submerged in the egg mixture.

Cover with foil and bake for 30 minutes.

Remove foil and bake for an additional 15-20 minutes until the top is golden brown and the center is set. The center must reach 165°F.

Let rest for 10 minutes before serving.