Preheat your oven to 350°F (175°C) and grease a medium-sized baking dish.

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

Press the mixture evenly into the greased baking dish and bake for 20 minutes.

While the meatloaf is baking, cook the macaroni according to package instructions, then drain.

In a saucepan, melt butter over medium heat, stir in flour to make a roux, then gradually whisk in milk until thickened.

Stir in paprika, dry mustard, and shredded cheese until melted. Adjust seasoning if needed.

Fold the cooked macaroni into the cheese sauce.

Pour the cheese-coated macaroni over the baked meatloaf and spread evenly. Add remaining cheese on top.

Bake for an additional 25 minutes until bubbly and golden.

Let the casserole cool for about 10 minutes before serving.