Wash the potatoes and cook in boiling water for 15-20 minutes. In the meantime, wash the cucumber and cut into small pieces. Wash the apple, quarter it and cut out the core.
Cut the apple quarters into thin slices. Wash and chop the dill. Mix cucumber, apple, dill, crème fraiche and quark. Season with salt, pepper and lemon juice. Drain and peel the potatoes. Serve with herring filet and sauce.
Garnish as desired with lettuce, apple slices and dill flags.