Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine ground beef, chopped onion, breadcrumbs, egg, milk, salt, and pepper. Mix thoroughly until fully combined and sticky. Press into a greased baking dish.

Bake the assembled meatloaf in the preheated oven for 20 minutes, until edges start to brown and the top firms up.

While the meatloaf is baking, bring a pot of salted water to a boil. Add macaroni and cook until al dente (about 7-9 minutes). Drain and set aside.

In a medium saucepan over medium heat, melt 1/2 cup of butter. Whisk in 1/4 cup of flour and cook for 1 minute to create a roux. Gradually whisk in 2 cups of milk, stirring until thickened (3-5 minutes).

Add paprika, dry mustard, and shredded cheese to the sauce, stirring until melted and smooth.

Carefully mix the drained macaroni into the cheese sauce until well coated.

Pour the macaroni and cheese mixture over the partially baked meatloaf, spreading evenly and sprinkle remaining cheese on top.

Return to the oven and bake for an additional 25 minutes, until bubbly and golden on top.

Let the casserole rest for 10 minutes before serving.