Preheat your oven to 350°F (175°C).

In a large bowl, combine ground beef, diced onion, breadcrumbs, egg, milk, salt, and pepper. Mix until smooth and well incorporated.

Press the meat mixture evenly into a greased 9x13-inch baking dish.

Place the dish in the oven and bake for 20 minutes.

While the meatloaf is baking, boil the macaroni until al dente, then drain and set aside.

In a saucepan over medium heat, melt the butter. Stir in the flour to create a roux, then gradually whisk in the milk until thickened, about 5-7 minutes.

Add salt, pepper, paprika, and dry mustard to the sauce. Fold in shredded cheddar cheese until creamy.

Gently stir the cooked macaroni into the cheese sauce until well coated.

Spread the mac and cheese mixture over the baked meatloaf and top with remaining shredded cheddar cheese.

Return the casserole to the oven and bake for an additional 25 minutes, or until the cheese is bubbly and golden brown.