Cook the potatoes unpeeled until they are firm to the bite. In the meantime, dice the onions and hold them in boiling water with a sieve for a minute. Mix the steamed onions with the finely chopped lovage, the spicy mustard, the fruit vinegar and a pinch of salt. Then stir in the sunflower oil spoon by spoon.
Peel the cooked potatoes after they have cooled down and put them in slices in a bowl, seasoning with a little salt and pepper in layers. Mix the dressing with the potatoes and let it steep for two hours.