Place the potatoes in a pan, cover with water and add the caraway and some salt. Bring to the boil and cook, covered, for 30 minutes until tender.
Drain the potatoes, then peel and thinly slice.
Mix the vinegar with the stock and mustard.
Season with salt and pepper.
Add a tablespoon of the potato slices and 3 tbsp oil to a blender and blitz finely. Season to taste.
Pour over the remaining potatoes and mix gently.
Wash and slice the radishes. Add to the potatoes with the cress. Leave to stand in the fridge.
Meanwhile peel and finely dice the onion. Brown in hot oil, then pour in the wine and allow to simmer. Spoon over the potato salad.