Preheat your oven to 175 degrees C (350 degrees F).

In a mixing bowl, combine the ground beef, diced onion, breadcrumbs, egg, 60 ml milk, salt, and pepper. Mix everything thoroughly until well combined.

Press the meat mixture into a greased baking dish, forming an even layer. Bake in the preheated oven for 20 minutes.

While the meatloaf is baking, cook the macaroni according to package directions. Drain and set aside.

For the cheese sauce, melt the butter in a saucepan over medium heat.

Stir in the flour and cook for about one minute, stirring continuously.

Gradually whisk in 480 ml of milk, continuing to cook until the mixture thickens.

Season the cheese sauce with salt, pepper, paprika, and dry mustard, stirring until well combined.

Add the shredded cheddar cheese to the sauce, stirring until melted and smooth.

Combine the cooked macaroni with the cheese sauce, mixing until the pasta is well coated.

Pour the cheesy macaroni mixture over the baked meatloaf layer evenly.

Top with the remaining cheese, then return to the oven and bake for an additional 25 minutes or until bubbly and golden.