Clean, wash and cut the beans into pieces. Cook in boiling salted water for about 10 minutes. Peel onion and cut into fine slices
Cut the bacon into strips and fry them crisply in a pan without fat. Remove. Steam onion in bacon fat until translucent. Deglaze with vinegar. Season with pepper, possibly salt and 1 pinch of sugar. Fold in the oil.
Drain the beans and mix with the marinade. Leave to stand for approx. 15 minutes. Wash and slice the tomatoes. Fold into the bean salad. Slice the cheese with a peeler directly over the beans. Sprinkle with bacon and garnish with chives