While the potatoes are boiling, wash the radishes, cut into thin slices, add a little salt and let stand. Wash the cucumber and slice it into fine pens on the vegetable slicer, add a little salt and leave to stand. Peel the onion and cut into fine cubes. Cut the basil leaves into fine strips or roughly chop. Heat the vegetable stock with the vinegar and mustard and stir until smooth.
When the potatoes are cooked, peel them immediately and cut them into fine slices. Pour salt over it, place the onions on top and then pour the warm vegetable-vinegar stock over them. Season with pepper and then pour the oil over it and carefully fold in everything. Let the potato salad soak for about 1/2 hour.
Then squeeze out the cucumber pieces with your hands and add to the potato salad.
Also squeeze out the radish slices and add to the salad. Carefully fold everything in again and let the potato salad steep for another 1/4 hour.
Just before serving, mix in the basil strips. If you add the basil to the salad at the end, it will retain its taste and aroma better.
This potato salad tastes good with all kinds of pan-fried foods.