Preheat your oven to 350°F (175°C) and prepare your baking dish by greasing it lightly.

In a mixing bowl, combine ground beef, finely chopped onion, breadcrumbs, egg, milk, salt, and pepper. Mix thoroughly and press into the bottom of the baking dish.

Bake the meatloaf base for 20 minutes until lightly browned.

Cook macaroni according to the package directions until al dente, about 7-8 minutes. Drain and set aside.

In a saucepan over medium heat, melt butter, stir in flour, and cook for about one minute. Gradually whisk in milk until thickened, about 3-5 minutes.

Remove from heat and stir in shredded cheese, paprika, and dry mustard. Mix until cheese is melted, then fold in drained macaroni.

Pour the cheesy macaroni mixture over the baked meatloaf base, spreading evenly.

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

Let it cool for 10 minutes before slicing and serving.