Preheat your oven to 350°F (175°C) and ensure your oven rack is in the center position.

In a large mixing bowl, combine ground beef, finely diced onion, breadcrumbs, egg, milk, and season with salt and pepper. Mix thoroughly until well combined.

Transfer the meatloaf mixture into a greased baking dish, pressing evenly. Bake for about 20 minutes until edges start to brown.

Cook macaroni in salted water according to package instructions until al dente, about 7–8 minutes. Drain and set aside.

In a medium saucepan, melt butter over medium heat, then whisk in flour. Cook for about one minute, stirring constantly.

Gradually add milk, whisking continuously until the sauce thickens, around 3–5 minutes.

Stir in paprika, dry mustard, and shredded cheese until melted and incorporated.

Mix the cooked macaroni into the cheese sauce until coated. Pour over the partially baked meatloaf and spread evenly.

Sprinkle remaining cheese over the top and return to the oven. Bake for an additional 25 minutes until cheese is melted and slightly golden.

Allow casserole to cool for at least 10 minutes before slicing and serving.