01 -
Mix sauce ingredients in a bowl, set aside.
02 -
Melt butter in skillet over medium heat. Cook sliced onions until caramelized (15-20 minutes). Remove onions and set aside.
03 -
Form beef into 4 thin patties. Season patties with salt and pepper.
04 -
Cook patties 3-4 minutes per side.
05 -
Butter one side of each bread slice.
06 -
Place 4 slices of bread butter-side down in the skillet. Layer: Swiss cheese, patty, onions, cheddar cheese. Top with remaining bread, butter-side up.
07 -
Grill until golden and cheese has melted (2-3 minutes per side).
08 -
Rest 1 minute before cutting. Serve with secret sauce.