For the dressing, mix the soured cream,
chives, mustard, 1 tbsp water and plenty
of seasoning, measure 2 tsp of the
vinegary juices from the onion, then stir
to combine. Taste and add more of the
vinegar if you like, then discard the rest.
Peel the skins off the potatoes if you like,
then cut them in half or into quarters and
toss with the dressing and onions.