Peel and finely dice the onions and fry them in 1 tablespoon of oil. Stir in mustard and deglaze with vegetable stock. Bring to the boil. Stir in vinegar, season with salt, pepper and sugar. Whip the rest of the oil into it. Pour the warm marinade over the prepared potatoes. You can also vary the sauce with finely diced gherkin, apple sticks, capers and finely chopped dill