Place the oil in a large skillet and heat over high heat. Add the onion and saute until the onion is soft and translucent. Add the tomatoes, salt and chile flakes and cook over medium-high heat, uncovered, 6 to 7 minutes, stirring occasionally. Add the vinegar, sugar, raisins and pine nuts and cook, stirring, another 2 minutes. Lower the heat to medium and add the fish in a single layer, if possible. Baste with the sauce and cook, covered, for 7 minutes. Uncover and simmer until the liquid is greatly reduced and the fish is in a thick, flavorful sauce. Sprinkle with parsley. Serve hot or at room temperature.