For the herring salad, mix all the ingredients for the marinade.
Peel the potatoes and cut into cubes. Cut the herring fillets into cubes as well. Peel apples and cut into quarters, removing the core. Cut apples, pickled cucumbers and onion into small cubes.
Place all ingredients in a glass or porcelain bowl and stir in the marinade. Cover and chill the herring salad for at least 1 hour – preferably overnight.