In large bowl, mix ground beef, breadcrumbs, onion, milk, egg, salt and pepper. Use two hands and mix well.
In a separate small bowl, mix ketchup, mustard and brown sugar.
Lightly grease a shallow baking pan (a loaf pan is ideal), and shape the beef mixture into a loaf. Cover with ketchup mixture and bake in oven for 45-55 minutes. Check with instant read thermometer. Internal temperature should be 160 degree F.