In a large bowl, combine the beef, egg, onion, milk and bread OR cracker crumbs. Season with salt and pepper to taste and place in a lightly greased 5x9 inch loaf pan, OR form into a loaf and place in a lightly greased 9x13 inch baking dish. In a separate small bowl, combine the brown sugar, mustard and ketchup. Mix well and pour over the meatloaf.