01 -
In a small bowl, stir together mayo, ketchup, pickle relish, and Worcestershire sauce. Set aside for later.
02 -
Heat 1 tablespoon butter in a big pan set on medium heat. Toss in the onion slices, stirring every so often, and cook until they're golden and soft—takes about 15-20 minutes. Take them off the heat once done.
03 -
Divide the ground beef into 4 thin patties, sprinkle evenly with salt and pepper. Keep the same pan hot on medium-high heat, and cook each patty for roughly 3-4 minutes per side till they're done. Set aside.
04 -
Spread a layer of butter on one side of each bread piece. Stack in this order: unbuttered side of bread down, a slice of Swiss, a patty, some cooked onions, a slice of cheddar, then a second bread slice with the buttered side facing up. Brown the sandwich in the skillet for 2-3 minutes per side until crispy outside and the cheese melts.
05 -
Let the sandwiches sit on the counter for a minute to cool slightly. Slice diagonally before serving. Offer the secret sauce on the side for dipping or drizzle some over the sandwich.