01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a large mixing bowl, combine the ground beef, finely chopped onion, minced garlic, breadcrumbs, egg, milk, ketchup, Worcestershire sauce, salt, and black pepper. Mix gently with your hands until just combined, being careful not to overwork the mixture.
03 - Press the meat mixture evenly into the bottom of the prepared baking dish, forming a uniform layer. Bake for 20 minutes until partially cooked through.
04 - While the meatloaf base 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. Continue cooking and whisking for 2 to 3 minutes until the sauce thickens slightly.
06 - Stir the shredded sharp cheddar, grated Parmesan, dry mustard, paprika, salt, and pepper into the sauce. Stir 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, panko breadcrumbs, and melted butter. Sprinkle this 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 brown and bubbling and the meatloaf layer is fully cooked through.
10 - Remove from the oven and let the casserole rest for 10 minutes before slicing. Serve hot.