01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a large bowl, combine the ground beef, chopped onion, minced garlic, breadcrumbs, egg, milk, ketchup, Worcestershire sauce, salt, and black pepper. Mix gently until just combined without overworking the meat.
03 - Press the meatloaf mixture evenly into the bottom of the prepared baking dish. Bake for 20 minutes until partially cooked through.
04 - While the meatloaf bakes, cook the elbow macaroni according to package instructions until just al dente. Drain well and set aside.
05 - In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute to form a roux. Slowly pour in the milk while whisking constantly and cook for 2 to 3 minutes until the sauce is slightly thickened.
06 - Stir the cheddar cheese, Parmesan cheese, dry mustard, paprika, salt, and pepper into the sauce until the cheeses are fully melted and the sauce is smooth. Fold in the cooked macaroni and mix until evenly coated.
07 - Spread the macaroni and cheese mixture evenly over the partially baked meatloaf layer in the baking dish.
08 - In a small bowl, combine the shredded cheddar cheese, breadcrumbs, and melted butter. Sprinkle the mixture evenly over the top of the casserole.
09 - Return the casserole to the oven and bake for 30 to 35 minutes until the topping is golden brown and the casserole is cooked through and bubbling.
10 - Remove from the oven and let the casserole rest for 10 minutes before slicing. Serve hot.