Heat closed medium-size contact grill for 5 minutes. Position drip tray to catch drippings.
Meanwhile, in large bowl, mix beef, onion, brown sugar, vinegar, Worcestershire sauce and 1/4 cup of the ketchup. Shape mixture into 4 patties, about 1/2 inch thick.
When grill is heated, place patties on grill. Close grill; cook 4 to 7 minutes or until thoroughly cooked and no longer pink in center.
Serve burgers in buns with lettuce, tomato and remaining ketchup.