Coat grill with cooking spray before starting the grill. In a bowl, combine the beef, 1 cup onions, Worcestershire and garlic powder; shape into six patties.
Grill, covered, over medium-high heat for 5 minutes per side or until a meat thermometer reads 165° and the juices run clear. Place 1 slice cheese on each burger. Cover and cook 1 minute or until cheese melts. Arrange burgers on rolls with lettuce and tomato. Spread mustard over cheese and top with remaining onions.