01 -
Stir everything for the sauce together in a small bowl, then leave it for later.
02 -
Heat a big skillet with some butter on medium flame. Toss in the onion slices and let them cook slowly for about 15-20 mins until sweet and golden. Set aside once done.
03 -
Make 4 thin patties out of the beef, then sprinkle salt and pepper to season them.
04 -
Pop the patties onto a hot skillet. Cook each side for about 3 or 4 minutes.
05 -
Spread butter on one side of every bread slice.
06 -
With the buttered side down on 4 bread slices, layer on: Swiss cheese, a cooked patty, onions, then cheddar cheese. Top it all with another buttered slice, butter-side facing up.
07 -
In the skillet, toast the sandwiches to a crispy golden, letting the cheese melt inside. Each side takes around 2 to 3 minutes.
08 -
Let your sandwiches rest for a minute before slicing them up. Use the sauce to dip or spread.