01 -
Combine mayonnaise, ketchup, sweet pickle relish, and Worcestershire sauce in a small bowl. Mix well and set aside.
02 -
Melt 1 tablespoon butter in large skillet over medium heat. Cook sliced onions, stirring occasionally, until caramelized, about 15-20 minutes. Remove and set aside.
03 -
Form beef into 4 thin patties, season with salt and pepper. Cook in same skillet over medium-high heat, 3-4 minutes per side. Set aside.
04 -
Butter one side of each bread slice. Layer bread (butter-side down), Swiss cheese, patty, onions, cheddar cheese, and top bread (butter-side up). Grill until golden and cheese melts, 2-3 minutes per side.
05 -
Let rest for one minute before slicing diagonally. Serve with secret sauce on side or drizzled over top.