Preheat oven to 400°F and grease a 12-cup muffin tin. Mix 1/2 cup ketchup, brown sugar and vinegar in a small bowl.
In a large bowl, combine ground beef, egg, onion, bread crumbs and remaining 1/2 cup ketchup; season with salt and pepper. Combine ingredients well.
Place 2 1/2 tsp. ketchup mixture in bottoms of prepared muffin wells. Form meat mixture into 12 equal balls and place in muffin tin, pressing down to smooth top of meat.
Cook meat loaves until no longer pink and juices bubble around edges, about 20 minutes. (Center of loaves should reach 160°F on a meat thermometer.) Remove from oven and let sit in pan for 5 minutes before carefully removing. Serve with mashed potatoes and peas and carrots, if desired.