In a large skillet over medium heat, cook the breakfast sausage until browned and fully cooked. Break it into small crumbles as it cooks. Add the diced onion and sauté until softened, about 3–4 minutes. Drain excess grease and set aside.

Grease a 9×13-inch baking dish. Spread the cubed bread evenly across the bottom. Sprinkle the cooked sausage and onion mixture over the bread. Add any optional vegetables at this stage.

Top evenly with the shredded cheeses.

In a large bowl, whisk together the eggs, milk, salt, pepper, garlic powder, and dry mustard. Whisk until fully combined.

Slowly pour the egg mixture over the casserole, making sure all bread pieces are moistened. Press down gently with a spatula if needed.

Cover tightly with foil or plastic wrap and refrigerate for at least 8 hours or overnight. This allows the bread to soak up the custard, creating a soft, fluffy interior when baked.

Remove the casserole from the refrigerator 30 minutes before baking.

Preheat oven to 350°F (175°C).

Bake covered with foil for 30 minutes.

Remove foil and bake an additional 20–25 minutes, until golden on top and set in the center.

Let rest for 10 minutes before slicing and serving.