01 -
Melt the butter over medium heat, then let the onions cook covered for about 5 minutes. Stir them every so often on low heat for 30-35 minutes until they're dark brown. Finish by adding the vinegar and scraping the pan.
02 -
Shape 3 burger patties and season them right before you cook. Heat up your griddle to about 375-400°F.
03 -
Using some oil, cook the patties for 3 minutes on each side.
04 -
Spread butter on the bread slices and toast each side on the griddle for 1-2 minutes until golden.
05 -
Lay cheese, caramelized onions, and a burger patty on toasted bread. Place another bread slice on top, press gently, and serve up.