Rinse fish well and dry them well on paper towels.

Soak the raisins in the wine for at least 30 minutes.

Heat oil in a 4-qt. pan over medium-high heat. Add onion; cook until browned, 10–12 minutes. Add vinegar, reduce heat to medium-low, and cook until soft, 6–8 minutes. Stir in raisins, nuts, and salt and pepper; let cool.

Drench the sardines in flour (do this by placing flour in a plastic bag. Add the fillets and shake the bag), fry them in hot oil for about 3 minutes or until slightly golden. Drain them well on paper towel and salt them.

When both the fish fillets and the onion marinade have cooled off, start layering them in a serving pan: start with a layer of onions, then a layer of fish, then again onions, fish, etc to end with the onions.

Seal with plastic wrap or foil, and refrigerate for al least 1 or 2 days before eating. It actually tastes even better after 4 or 5 days.