01 -
Stir mayonnaise, ketchup, relish, and Worcestershire in a small dish. Put aside when blended.
02 -
Heat a tablespoon of butter in a big pan over medium. Stir onions now and then as they cook down and brown over 15-20 minutes. Set them aside after they're ready.
03 -
Shape the beef into four flat patties and sprinkle with salt and pepper. Using the same pan, cook them over medium-high heat, flipping after 3-4 minutes each side. Put them aside for now.
04 -
Spread butter on one side of the bread slices. Stack like so: bread (buttered side out), Swiss cheese, cooked patty, caramelized onions, cheddar cheese, bread (buttered side out). Grill each side until bread's toasty and cheese melts, around 2-3 minutes.
05 -
Wait about a minute before cutting the sandwiches into triangles. Add the sauce on the side or drizzle over the top.