Wash the cucumber, grate dry and cut in half crosswise. Cut the cucumber halves into thin longitudinal strips with a slicer or slicing machine. Wash the dill and dab dry. Roughly chop the flags of 2 stems. Mix sour cream, oil and vinegar and season with salt and pepper. Arrange cucumber slices, sour cream and prawns on 4 plates and garnish with dill flags. Spread pumpernickel with fat and cut in half diagonally