Lay the fish in a dish and cover with cold water to extract the salt.
Leave in the refrigerator for 12 hours, changing the water three times.
Drain and skin the herrings then cut from the head along the backbone and open out both sides.
Remove the fins.
Place the fillets in a bowl and add the vinegar and sugar; marinate for 2 hours.
Core and slice the apples and sprinkle with lemon juice.
Slice the onions and pickles.
Mix the apple, onion and pickles with the herring fillets.
Spoon over the sour cream and chill for 30 minutes before serving.