Brush the potatoes thoroughly in water. Put them in a pot with plenty of water and bring to the boil. Cover and cook for 20 minutes. In the meantime, clean and wash the radishes and, except for a few for garnishing, cut them into fine sticks.
Wash the cucumber, quarter it lengthwise and cut into thin slices. Wash chives and dill, dab dry and chop the dill finely, except for a few flags for garnishing. Cut the chives into fine rolls.
Stir the quark, crème fraîche and yoghurt until smooth. Stir in the cucumber radishes, dill, chives and prawns, except for one or two tablespoons. Season with salt and pepper. Sprinkle the finished quark with remaining ingredients and serve garnished with dill and radishes.