Preheat broiler. Melt butter in medium saucepan over medium heat; stir in Worchestershire sauce, garlic salt, and mustard. Cook for 2 minutes, stirring often; remove from heat. Brush onion halves with sauce and place on broiler pan. Brush chicken with sauce and place, skin side up, on top of onion halves. Arrange oven rack so that chicken is about 6 inches from heat. Broil for 15 minutes; turn onion and chicken skin side down, leaving onion on top. Brush liberally with sauce; broil for 15 minutes longer, or until chicken is fork tender. Remove from oven. Serve hot sprinkled with salt and pepper to taste.
