Thaw the fish and cut the fillets into 1/2-inch strips.
Store in a refrigerator for at least two days—longer won’t hurt.
Drain, and cover the fish with white vinegar for another day.
Bring to a boil.
Place in a refrigerator until cool.
Alternate layers of fish and onion slices in a crockery pot or wide-mouthed gallon jar (or smaller jars), pour brine over the fish, cover, and refrigerate for two weeks.