Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine the ground beef, finely chopped onion, breadcrumbs, egg, milk, and a generous pinch of salt and pepper. Mix until well combined but avoid overmixing.

Transfer the meatloaf mixture into a greased baking dish, pressing it down evenly.

Place the baking dish in the preheated oven and bake the meatloaf for about 20 minutes.

Bring a pot of salted water to a boil, then add the macaroni and cook until al dente, usually about 7-9 minutes. Drain and set aside.

In a medium saucepan, melt the butter over medium heat. Stir in the flour and cook for about 1 minute to create a roux, then gradually whisk in the milk until it thickens and bubbles.

Stir in salt, pepper, paprika, and dry mustard, then add the shredded cheese, mixing until fully melted and creamy.

Gently stir the cooked macaroni into the cheese sauce, then pour the mixture over the partially cooked meatloaf, spreading it evenly.

Sprinkle any remaining shredded cheese on top of the macaroni layer.

Return the dish to the oven and bake for an additional 25 minutes until the cheese is melted and bubbly.

Remove the casserole from the oven and let it rest for about 10 minutes before serving.