Mix ground beef, Worcestershire sauce, salt, pepper and garlic powder with your hands in a large bowl.
Form beef mixture into six equal-sized balls; set aside.
Melt butter in a large skillet over medium heat; add onions; cook until carmelized, stirring continously (~15 minutes); remove onions; set aside.
Flatten each meat ball in skillet; fry until browned on both sides; put a slice of bread under each beef patty; distribute carmelized onions equally on each beef patty; followed by a cheese slice.
Spread mustard on remaining bread slices; place mustard-side down on each patty; flip melts; brown second side (~two minutes).
