01 -
Stir together Worcestershire sauce, ketchup, mayo, and relish in a small mixing bowl. Keep it aside for now.
02 -
Heat a large pan over medium heat with a small pat of butter. Toss in the sliced onions and cook them gently, stirring from time to time, until they're golden and soft, which takes about 15 to 20 minutes. Once done, set them aside.
03 -
Split the beef into four pieces and mold each one into a thin patty. Sprinkle both sides with salt and pepper to bring out the flavor.
04 -
Using the same pan, cook the patties over medium-high heat for 3 to 4 minutes per side until done. Take them out of the pan and let them rest.
05 -
Spread butter on one face of each slice of rye bread. In a clean pan or on a griddle over medium heat, lay down 4 slices, butter-side touching the surface.
06 -
Stack onto each slice of bread a piece of Swiss cheese, then add a beef patty, caramelized onions, a cheddar cheese slice, and top it all with another buttered bread slice, placing the buttered side up.
07 -
Cook the sandwiches on the pan or griddle over medium heat. Flip them over after 2 to 3 minutes, or once the bread is toasty and the cheese is melty.
08 -
Take the melts off the stove and let them sit for a minute. Cut them in half and serve hot with the secret sauce, either on the side or drizzled on top.