Wash potatoes thoroughly and cook in boiling salted water for about 20 minutes. Then drain, rinse and peel. Cut the potatoes into slices. In the meantime, heat the stock and then pour it over the potato slices.
Let it cool down. In the meantime clean and wash spring onions and radishes. Cut spring onions into rings, radishes into quarters. Wash the dill, dab dry and cut finely. Mix potatoes, spring onions and radishes.
Mix sour cream, vinegar and yoghurt. Season with salt, pepper and sugar and stir in 2/3 of the dill. Pour dressing over the potatoes, fold in carefully and leave to stand for approx. 30 minutes.
Arrange the salad with one matie on each of four plates. Sprinkle with the remaining dill and garnish with a slice of lemon if desired.