Soak the salted herrings for 12-24 hours and then wash them thoroughly several times to loosen the flakes. Cut off the heads and peel off the inner black skin. Now wash the herrings again and then layer them with the sliced onions and the spices in a small stone pot. Mix the vinegar with the boiled, cooled water and pour the marinade over the herrings. Let it steep for 2-3 days.