Wash potatoes thoroughly and cook in boiling water for about 25 minutes. Drain, rinse and peel. Leave to cool slightly and then cut into slices.
Peel and finely chop the onion. Bring stock and onion to the boil, simmer for 1-2 minutes. Dice cucumbers. Measure out 100 ml cucumber water. Pour boiling broth over potato slices and let them simmer for about 30 minutes, stirring several times.
For the sauce, mix sour cream, mayonnaise and 100 ml cucumber water. Season to taste with salt, pepper and sugar. Wash the chives, shake dry and cut into fine rolls.
Mix potatoes, cucumber, 3/4 of the chives and sauce well. Season again with salt and pepper. Leave to stand for about 45 minutes. Arrange in a bowl sprinkled with remaining chives.