How to Make It
In a small bowl, soak the raisins in warm water to cover. Cut the sole fillets in half lengthwise, then cut them into 3-inch strips.
Pour 1/2 inch of vegetable oil into a large nonreactive skillet and turn the heat to high. Pat the fish dry with paper towels, then turn them lightly in the flour and shake off the excess. When the oil is hot, fry the sole until deep gold on both sides. Using a slotted spatula, transfer the fish to paper towels; sprinkle with salt. Discard the oil and wipe out the skillet.
Add the olive oil, onion, sugar and a large pinch of salt to the skillet. Cover and cook over low heat, stirring occasionally from time to time, until the onion is completely limp but still pale, 15 to 20 minutes. Uncover and cook over moderate heat until the liquid boils off and the onion is a light nut brown, about 3 minutes. Add the vinegar and boil over high heat for 3 minutes. Remove from the heat and let cool.
Drain the raisins and pat them dry. Place the fried sole in a ceramic or glass dish that will hold is snugly. Using a slotted spoon, spread the onion over the fish. Sprinkle the pine nuts and the raisins on top and then pour the remaining liquid from the pan over all. Cover with plastic wrap and set aside at room temperature for up to 12 hours or refrigerate for up to 5 days. Let the fish come to room temperature before serving.