01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking dish.
02 - In a large bowl, combine ground beef, onion, garlic, breadcrumbs, egg, milk, ketchup, Worcestershire sauce, salt, and pepper. Mix gently until just combined, being careful not to overwork the meat.
03 - Press the meat mixture evenly into the bottom of the prepared baking dish. Bake for 20 minutes.
04 - While the meatloaf layer bakes, cook the elbow macaroni according to package instructions until just al dente. Drain 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 continue cooking for 2 to 3 minutes until the sauce thickens slightly.
06 - Stir the cheddar and Parmesan cheeses, dry mustard, paprika, salt, and pepper into the sauce. Once the cheese is fully melted and the sauce is smooth, fold in the cooked macaroni and mix until evenly coated.
07 - Spread the macaroni and cheese evenly over the partially baked meatloaf layer in the baking dish.
08 - In a small bowl, combine the shredded cheddar cheese, panko 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 top is golden and bubbling and the casserole is cooked through.
10 - Allow the casserole to cool for 10 minutes before slicing. Serve hot.