Preheat your oven to 350°F (175°C). In a large mixing bowl, combine ground beef, finely chopped onion, breadcrumbs, egg, milk, salt, and pepper. Mix thoroughly and transfer to a greased baking dish.

Bake the meatloaf in the preheated oven for 20 minutes, until the edges firm up and a light golden color forms.

While the meatloaf bakes, cook the macaroni according to package directions until al dente, then drain.

In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in ¼ cup of flour and cook for about 1 minute. Gradually add in 2 cups of milk, stirring until thickened, about 3-5 minutes. Mix in shredded cheese, salt, pepper, paprika, and dry mustard until melted.

Add the drained macaroni into the cheese sauce, stirring to coat well.

Pour the macaroni and cheese mixture over the baked meatloaf, spreading evenly. Sprinkle any remaining cheese on top. Bake for an additional 25 minutes until the cheese is melted and bubbly.

Let the casserole rest for about 10 minutes before slicing.