Wash the prawns, dab dry. Heat the oil in a pan and fry the prawns for about 2 minutes. Season with salt and pepper. Leave to cool.
Peel and finely chop the onion. Boil up the onion, lemon juice and sugar in a small pot and simmer for 3-4 minutes. Remove from the heat and let it cool down.
For the sauce, mix mayonnaise, sour cream and cucumber stock. Season to taste with salt and pepper.
Cut the bismarck herring and matie into pieces. Finely dice the cucumbers. Peel apple, quarter it, remove core and dice quarter. Wash parsley and dill, shake dry and chop finely. Mix all the prepared ingredients and crabs with the sauce, possibly adding lemon wedges.