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

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

Transfer the meatloaf mixture to the greased baking dish and press it down evenly. Bake for 20 minutes.

Cook the macaroni according to package directions, about 7-8 minutes, then drain.

Melt butter in a saucepan, stir in flour to make a roux, gradually add milk, then add paprika, dry mustard, and shredded cheese until smooth.

Add the cooked macaroni to the cheese sauce and stir to coat.

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

Sprinkle more shredded cheese on top and bake for another 25 minutes until bubbly and golden brown.

Allow to rest for 10 minutes before slicing and serving.