01 -
In a small bowl, mix together mayonnaise, ketchup, relish, and Worcestershire sauce. Once it's blended, set it aside.
02 -
Add a tablespoon of butter to a big skillet and warm it over medium heat. Stir the sliced onions every now and then until golden and sweet, about 15-20 minutes. Move them out of the skillet when done.
03 -
Shape the beef into four thin patties. Season each one with some salt and pepper, then fry them in the same skillet over medium-high heat for roughly 3-4 minutes per side. Once done, set them aside for now.
04 -
Spread butter on one side of each bread slice. Put one slice butter-side down, add Swiss cheese, a beef patty, onions, cheddar cheese, and another slice with the butter side up. Grill each sandwich for about 2-3 minutes on each side till the bread is golden and the cheese melts.
05 -
Let the sandwiches sit for a minute. Cut them in half diagonally and either drizzle sauce on top or serve it on the side.