01 -
Cook onions in butter on medium heat covered 5 minutes, then stir occasionally on low 30-35 minutes until dark brown. Deglaze with vinegar
02 -
Form 3 patties, season just before cooking. Heat griddle to 375-400°F
03 -
Cook patties in oil 3 minutes per side
04 -
Butter bread slices, toast on griddle 1-2 minutes per side
05 -
Top toasted bread with cheese, onions, and burger patty. Close sandwich and serve