For the sauce, wash and quarter apples and remove the core. Put two apple quarters aside. Cut the rest into thin slices. Mix mayonnaise and yoghurt. Fold in apple slices.
Clean, wash and cut the spring onions into rings. Fold onion rings and peppercorns into the sauce. Season to taste with salt, sugar and a little mustard. Peel and wash the potatoes, cut into slices and dab dry with kitchen paper.
Heat the oil in a frying pan. Fry the potatoes for about 15 minutes at medium heat, turning them over. Season with caraway seeds and salt. In the meantime cut cucumber and apple quarter into long thin sticks. Drain Bismarck herrings and cut them in half lengthwise.
Cover with apple and cucumber strips and roll up. Arrange fish with fried potatoes and some sauce. Remaining sauce extra is enough.