Prepare the salt herrings as follows: Cut off the heads and discard. Slit the fish from head to tail. Remove entrails and scrape the inside well. Cut off tail and fins. Rinse the fish thoroughly in cold water. Scrape and pull off the bluish skin. Be careful not to tear outside of fish. Remove backboe and as many bones as possible without breaking the flesh.
Let fillets soak in cold water in a large bowl for 2 hours. While fish is soaking, slice onion in to thin rings. Mix together the vinegar, water and bay leaf.
Drain herring and cut into pieces about 2½ inches square. Put a layer of herring into a shallow baking pan and top with some of the onion rings. Repeat the layers until all the fish and onion rings are used. Pour the vinegar and water mixture over it. Chill overnight in refrigerator to blend the flavors thoroughly.
When ready to serve, drain off the liquid. Toss the herring and onion rings lightly to mix. Garnish wiht chopped parsley.