01 -
Stir everything for the sauce in a bowl, then leave it to rest.
02 -
Drop some butter in a pan and cook onions on medium heat. Stir now and then until the onions turn golden, around 15-20 minutes. Take them out when done.
03 -
Divide the beef into four thin rounds. Sprinkle with a little salt and pepper.
04 -
Cook each patty for about 3 or 4 minutes, flipping halfway.
05 -
Spread butter on one side of each piece of bread.
06 -
Lay 4 bread slices in the pan with the buttered side down. Stack them up like this: Swiss cheese, a patty, those onions, cheddar cheese. Finish with the other slices of bread, butter side facing up.
07 -
Cook the sandwiches until the outsides turn golden and the cheese becomes gooey, about 2 or 3 minutes on each side.
08 -
Wait a minute before slicing. Offer it with the sauce alongside.