Rinse the herring fillets under running water for 1-2 minutes. Taste a small piece to check if it's still too salty. If needed, place the fillets in a bowl with mineral water or milk and refrigerate for 30 minutes.
In a large bowl, combine the cream, sour cream, mayonnaise, and cucumber liquid. Season with sugar and freshly ground pepper (salt is usually not needed as the fish will release enough salt into the sauce).
Wash, peel, and core the apples. Cut them into thin slices. Cut the pickled cucumbers into thin slices. Peel the onions and cut into fine rings.
Add the apple slices, cucumber slices, and onion rings to the sauce and stir to combine.
Fold the herring fillets into the sauce. Cover and refrigerate for 2 hours.
Garnish with fresh dill and serve with jacket potatoes or wholemeal bread.