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

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

Bake the meatloaf in the oven for approximately 20 minutes until it starts to firm up.

Cook the macaroni according to package directions until al dente, typically about 7-9 minutes. Drain and set aside.

In a saucepan, melt butter, whisk in flour to create a roux, and gradually add milk, stirring until thickened. Mix in salt, pepper, paprika, mustard, and shredded cheese until creamy.

Combine the cooked macaroni with the cheese sauce and pour over the partially baked meatloaf, spreading evenly.

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

Let the casserole rest for about 10 minutes before serving.