Cook the potatoes in salted water for about 20 minutes, then peel them. While the potatoes are boiling, stir the dressing in a large salad bowl.
Mix together mayonnaise, yoghurt, vinegar, mustard and 4 tablespoons of cucumber water. Peel the onion and cut into fine cubes. Then sweat the onion cubes in a little oil in the pan, then add them to the sauce.
Dice the cucumber and tomato, cut the chives into fine rings and fold everything into the sauce. Finally, season the dressing with salt and pepper.
Cut the potatoes into slices about 5 mm thick and immediately add them to the salad dressing. So the panes can pull through easily. Cover and chill the potato salad for about 2 hours.