01 -
Combine all sauce ingredients in a bowl and stir till smooth. Put aside for later
02 -
Slowly cook the onion slices with butter on medium heat until they turn soft and golden, roughly 15-20 minutes
03 -
Shape beef into 4 flat patties, add salt and pepper, then cook about 3-4 minutes on each side
04 -
Spread butter on each bread slice. Stack Swiss cheese, beef patty, cooked onions, and cheddar between two slices
05 -
Heat sandwiches in pan until they're crispy and the cheese gets melty, around 2-3 minutes per side