Wash the fish and dab dry. Boil egg in boiling water for about 9 minutes until hard. In the meantime, wash parsley, dab dry and chop finely, except for something to garnish.
Wash and drain the raisins. Quench the egg cold, peel it and crush it finely with a fork. Mix cream, horseradish, egg, lemon juice and parsley and season to taste with salt, pepper and sugar.
Fold the raisins into the horseradish cream. Arrange 2 herrings on each plate, pour some sauce over the fish, serve with the remaining parsley and, if desired, garnished with lemon slices.
The rest of the sauce is extra.