Put eggs in boiling water and simmer for 9-10 minutes. Drain and quench. In the meantime mix yoghurt, sour cream, mineral water and mustard. Season to taste with salt, pepper and sugar.
Clean and wash the salad, pluck into bite-sized pieces and drain well. Clean, wash and cut the rocket into coarse strips. Clean, wash and quarter radishes. Clean, wash and slice the mushrooms.
Peel and halve the eggs. Mix salad, rocket, radishes and mushrooms, carefully fold in eggs. Arrange on a plate, pour a dash of dressing over it. Add the rest of the dressing separately.