Soak the herrings well overnight, then fillet them. Also peel off the skin for the taste. Put the fillets in a sealable container (bowl with lid). Cut the onions into rings, cut the cucumber into small pieces and add to the herring fillets. Add all the spices and finally pour chilled buttermilk over everything. The whole thing has to steep in the refrigerator for at least 12 hours.
Then season to taste and, if necessary, season with vinegar, or better with the cucumber broth.