01 -
Mix the mayo, ketchup, pickle relish, and Worcestershire sauce together in a small bowl. Stir till it’s nice and smooth, and then set it aside.
02 -
Throw a tablespoon of butter into a big pan over medium heat. Toss in the onion slices and cook them, stirring here and there, until they’re golden brown—this’ll take 15 to 20 minutes. Take them out and put them aside.
03 -
Shape the beef into four thin patties, add some salt and pepper, and throw them into the same pan at medium-high heat. Let them cook for 3 or 4 minutes on each side. Once they’re done, set them aside.
04 -
Spread butter on one side of the bread slices. Start building sandwiches layer by layer: bread with buttered side down, Swiss cheese, a beef patty, caramelized onions, cheddar cheese, and top it off with another buttered bread slice, buttered side facing up. Grill for 2 to 3 minutes on each side until the outsides are golden and the cheese is gooey.
05 -
Let them cool for a quick moment (about a minute). Cut diagonally and serve with the secret sauce either on the side or poured on top.