01 -
Over medium heat, let onions cook in butter covered for about 5 minutes. Drop the heat to low and stir them now and then for 30 to 35 minutes. They should turn a deep brown before adding vinegar to deglaze.
02 -
Get 3 burger patties ready and wait to season them until you're about to cook. Set your griddle to reach 375–400°F.
03 -
Sear patties in the oil for 3 minutes, then flip and cook the same on the other side.
04 -
Smear butter across the bread slices, then place them on the griddle to toast for about 1 to 2 minutes per side.
05 -
Put cheese, the caramelized onions, and a patty on top of each toasted bread slice. Put another slice of bread over the top. Serve hot.