Boil the potatoes until they are done, peel them, let them cool and cut into small pieces. Cut hard-boiled eggs into small pieces as well. Drain the cucumbers, catch the stock, also cut. Cut the spring onions into rings, mix everything.
For the sauce, mix the sour cream and sour cream with mustard, some of the cucumber water, pepper and salt and mix with the salad ingredients.
Cover and leave to stand in the refrigerator for at least an hour.