Preheat your oven to 350°F (175°C) and prepare your baking dish by greasing it lightly.

In a large mixing bowl, combine the ground beef, finely chopped onion, breadcrumbs, egg, milk, salt, and pepper; mix until evenly combined.

Transfer the meat mixture into the greased baking dish, pressing it down to form an even layer. Bake for 20 minutes until starting to firm up and brown.

While meatloaf bakes, boil salted water, add macaroni, and cook until al dente, usually about 8-10 minutes. Drain and set aside.

In a saucepan, melt butter, stir in flour to create a roux; cook for about 1 minute. Gradually whisk in milk, cooking until thickens (3–5 minutes). Season with salt, pepper, paprika, and dry mustard.

Remove cheese sauce from heat, stir in shredded cheese until melted. Fold in cooked macaroni until well coated.

After the meatloaf has baked for 20 minutes, pour the macaroni and cheese mixture over the meatloaf, spreading it evenly.

Return the casserole to the oven and bake for an additional 25 minutes until golden brown and bubbly.

Let the casserole rest for about 10 minutes before serving.