Wash the potatoes and cook in boiling water for about 25 minutes. Drain, rinse and peel. Let the potatoes cool down a little and then cut them into slices.
Peel and finely chop the onion. Bring 150 ml water to the boil, stir in the stock. Add onion, simmer for 1-2 minutes. Pour the hot onion stock over the potato slices and let them stand for about 30 minutes, stirring carefully several times.
Dice cucumbers. For the sauce, mix 100 ml cucumber water from the glass, sour cream and mayonnaise. Season vigorously with salt, pepper and sugar. Wash chives, shake dry and cut into fine rolls.
Mix potatoes, cucumbers, 3⁄4 of chives and cream sauce well. Leave to stand for approx. 45 minutes. Season potato salad with salt and pepper again. Arrange in a bowl. Sprinkle with remaining chives.