Gather the ingredients. Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x5-inch loaf pan.

Combine ground beef, onion, milk, bread crumbs, and egg in a large bowl, and season with salt and pepper. Press mixture evenly into the prepared loaf pan.

Stir ketchup, brown sugar, and mustard together in a small bowl until smooth, and spread evenly over the meatloaf.

Bake in the preheated oven until no longer pink in the center, about 1 hour. An instant-read thermometer inserted into the middle reads at least 160 degrees F (70 degrees C).

Serve hot and enjoy!