Preheat your oven to 350°F (175°C).

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

Press half of the mixture into a greased 9x13-inch baking dish and bake for 20 minutes.

Cook macaroni in boiling salted water until just al dente, about 7-8 minutes. Drain and set aside.

In a saucepan over medium heat, melt butter, stir in flour to form a paste, and gradually whisk in milk until thickened, about 3-5 minutes.

Stir in shredded cheese, paprika, and dry mustard until melted and smooth.

Fold cooked macaroni into cheese sauce and pour over the baked meatloaf.

Top with remaining cheese and bake for an additional 25 minutes.

Let rest for 10 minutes before slicing.