Preheat the oven to 350°F (175°C).

In a large mixing bowl, combine the ground beef, onion, breadcrumbs, egg, milk, salt, and pepper until well incorporated.

Grease a baking dish and press the meat mixture evenly into the bottom. Bake for about 20 minutes.

Cook the macaroni in boiling salted water until al dente, typically 7-9 minutes. Drain and set aside.

In a saucepan, melt the butter over medium heat, whisk in flour to create a roux. Gradually add milk until thickened, then stir in paprika, dry mustard, and cheese until melted.

Mix the cooked macaroni with the cheese sauce until fully coated.

Carefully pour the cheesy macaroni mixture over the meatloaf in the baking dish.

Sprinkle remaining shredded cheese on top and bake for an additional 25 minutes until bubbly and golden.

Let the casserole rest for about 10 minutes before slicing and serving.