01 -
Stir together mayo, ketchup, relish, and Worcestershire in a small bowl. Put it off to the side.
02 -
Melt butter in a pan and slowly cook the onion slices over medium heat until they’re golden, about 15 to 20 minutes.
03 -
Shape the beef into 4 thin burger patties, sprinkle with salt and pepper, and fry for 3-4 minutes on each side.
04 -
Take the rye bread, butter one side of each slice, and stack ingredients like this: Swiss cheese, a burger patty, caramelized onions, and cheddar cheese. Sandwich them together with unbuttered sides facing inward.
05 -
Place the sandwiches on a hot skillet, grilling until crispy and golden outside, and the cheese starts to ooze—about 2-3 minutes on each side.