Wash potatoes thoroughly and cook in boiling water for 20 minutes. In the meantime, put the matie in cold water. Clean and wash spring onions and cut into rings. Clean, wash and quarter the radishes.
Mix sour cream, yoghurt and lemon juice, season to taste with salt, pepper and sugar and fold in radishes and spring onions. Wash the dill, dab dry, put some dill flags aside for garnishing.
Finely chop the remaining dill. Drain potatoes, rinse with cold water and peel. Sprinkle with chopped dill. Arrange the dill potatoes, matie and sauce on plates and serve with the dill flags and, if desired, garnished with lemon wedges.