Skin and fillet fish. Don't worry about bones left in, they will dissolve. Cut fillets in bite size. Soak in salt brine 24 hours. Drain, but do not rinse. Next soak in vinegar brine for 24 hours. Drain, do not rinse. Pack fish with sliced onions in large bowl with final brine, cover and refrigerate for 7 days. After 7 days pack fish and onion in pint jars. Strain liquid through cloth and fill packed jars. Keep refrigerated. Will keep indefinitely.