Put herrings in cold water and let soak 12 hours. Dry fish well between paper towels. Skin, bone and cut into 1-inch lengths.
Place a layer of herring in a large mixing bowl. Cover with a layer of sliced onions. Repeat until all the fish is used.
Cover with a solution made of 2 parts vinegar to 1 part water. Chill for at least 12 hours.
Remove fish from vinegar solution when ready to serve. Garnish with sliced onions and sprigs of parsley. Will keep for several weeks refrigerated.