Preheat oven: Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking dish.
Make meatloaf: In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, milk, onion, garlic, salt, and pepper. Mix well. Shape into a loaf and place in the prepared baking dish.
Make mac and cheese: In a large saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute. Gradually whisk in the milk until smooth. Bring to a simmer and cook for 2-3 minutes, or until thickened. Stir in the Parmesan cheese, salt, and pepper.

Combine: Pour the mac and cheese over the meatloaf.