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 should read at least 160 degrees F (70 degrees C).
Serve hot and enjoy!
Notes
You can use crushed crackers instead of bread crumbs for added flavor and texture.