Heat 1 cup oil to 375 degrees F. in a deep saucepan.
Season the sole fillets with salt and pepper. Dredge the fillets in the seasoned flour and fry in the hot oil until golden brown. Remove and drain on paper towels.
In a 12 to 14-inch saute pan, heat the remaining 1/4 cup oil over medium heat. Add the onions and cook until softened and just light brown, 10 to 12 minutes. Add the currants, sugar, pine nuts, and vinegar and bring to boil. Cook 5 to 6 minutes until currants are soft, then allow to cool. When all the ingredients are cool, choose an earthenware dish that just fits the fried fillets, slightly overlapping. Top with the sweet and sour mixture and refrigerate for 24 hours. Serve at room temperature.
Recipe copyright 2001, Mario Batali. All Rights Reserved