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

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

Bake the meatloaf for about 20 minutes until starting to firm up.

Cook the macaroni according to package directions until al dente, then drain.

In a saucepan, melt the butter over medium heat, stir in the flour and cook for 1 minute. Gradually whisk in the milk until thickened. Add shredded cheese and stir until smooth.

Combine the cooked macaroni into the cheese sauce, stirring to coat.

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

Bake for an additional 25 minutes until cheese is bubbly and golden brown.

Let it rest for 5-10 minutes before slicing and serving.