Wash potatoes thoroughly and cook in boiling water for 20-25 minutes. Then drain, rinse and peel. Let potatoes rest for about 10 minutes.
For the vinaigrette, peel and finely dice the onions. Bring the onions, stock, vinegar, sugar, salt and pepper to the boil and simmer at low to medium heat for 10-12 minutes. Remove from the heat and fold in the oil.
Cut the potatoes into slices. Cut gherkins into slices. Wash and peel the cucumber, cut in half lengthwise and remove the seeds with a spoon. Halve cucumber again lengthwise and cut into pieces. Wash chives, shake dry and cut into fine rolls.
Mix the potatoes, gherkins, chives and vinaigrette. Arrange in a bowl.