Slice onions into thin half-moons. Combine vinegar and raisins in a measuring cup. Rinse sardines well inside and out, dry with paper towels, and roll in flour, shaking off excess.
Heat 3 tbsp. oil in a large frying pan over medium heat. Fry sardines in 2 batches, 1 1/2 minutes per side, being careful not to overcook. Transfer to a large shallow serving dish or platter and sprinkle with 1/2 tsp. salt (you can layer the sardines if necessary).
Wipe pan clean with paper towels, return to heat, and add remaining oil. When oil is hot, add onions and remaining 1/2 tsp. salt. Cook, stirring often, until onions are a soft, browned mass, 25 to 30 minutes.
Add vinegar and raisins and cook, stirring occasionally, 10 minutes. Pour over sardines and sprinkle with pine nuts. Let marinate for at least 1 hour at room temperature, covered, or chill for up to 4 days. Sprinkle with parsley to serve.
*Preorder cleaned sardines from your fishmonger.
Make ahead: Up to 3 days, covered and chilled.
Note: Nutritional analysis is per serving.