01 -
Slice onions thinly, then cook in melted butter over medium heat for 5 minutes covered. Reduce heat to low, stir every few minutes, and cook another 30-35 minutes until golden and sticky. Stir in vinegar at the end.
02 -
Shape the mixture into 3 even patties. Just before cooking, sprinkle with seasoning. Preheat a griddle to 375-400°F.
03 -
Add a little oil to the hot griddle, then cook patties for 3 minutes before flipping to cook the other side.
04 -
Spread butter on each bread slice, then heat on a griddle for 1-2 minutes per side until crisp.
05 -
Layer each toasted bread slice with some cheese, a portion of caramelized onions, and a burger patty. Top with another slice of bread, press gently, and enjoy.