Wash lettuce, shake dry and tear lightly. Wash and slice the tomatoes. Peel onions and cut into rings.
Heat 2 tablespoons of oil in a large frying pan and fry the onions for about 5 minutes. After about 3 minutes season with salt and 1 teaspoon sugar, deglaze with vinegar.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Remove the onions, heat 1 tbsp. oil in the pan. Fry the frozen burgers for approx. 10 minutes while turning. After approx. 8 minutes spread cheese on the burgers.
In the meantime, cut the rolls in half and bake in a hot oven for about 6 minutes. Cover rolls with prepared ingredients and sauce.