Preheat your oven to 350°F (175°C).

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

Transfer the meatloaf mixture into a greased baking dish and bake for 20 minutes.

Cook macaroni in salted boiling water until al dente, then drain.

In a saucepan, melt butter, whisk in flour and cook for 1 minute. Gradually add milk and whisk until sauce thickens.

Add shredded cheese, salt, pepper, paprika, and dry mustard to the cheese sauce until smooth.

Combine the cooked macaroni with the cheese sauce, stirring to coat evenly.

Pour the macaroni and cheese mixture over the baked meatloaf and spread evenly.

Return the casserole to the oven and bake for another 25 minutes until cheese is melted and bubbly.

Let the casserole rest for about 10 minutes before slicing and serving.