01 -
Stir together mayo, ketchup, relish, and Worcestershire sauce in a bowl.
02 -
Heat butter and cook onions until they caramelize, about 15-20 minutes.
03 -
Season the meat and shape it into 4 thin patties.
04 -
Cook patties for 3-4 minutes per side.
05 -
Spread butter on the bread, then layer the cheese, meat, and onions.
06 -
Toast sandwiches on both sides for 2-3 minutes to melt the cheese.