In a 12 to 14-inch saute pan over medium heat, heat 1 tablespoon olive oil until hot but not smoking. Add the fish pieces skin side down and cook until the fillets are almost cooked through and the skin side is a light golden brown, about 3 minutes. Turn the fish and cook until light golden brown on the other side. Remove the fish from the heat and allow to drain on a paper towel.
In a heavy-bottomed 4-quart pot, over medium heat, combine the remaining olive oil, onion slices, raisins, and white wine vinegar. Let the mixture simmer gently for 10 minutes. Remove from the heat, and allow to come to room temperature.
Lay the fish pieces in an airtight container deep enough so the olive oil mixture can cover the fillets. When the fillets and the mixture are both at room temperature, pour the olive oil mixture over the fillets. Submerged in the airtight container, the fish pieces should be left to marinate in the refrigerator several days before being served.