1 (28-oz) can whole tomatoes in purée
1 medium onion, chopped
2 tablespoons olive oil
1 tablespoon tomato paste
2/3 cup packed dark brown sugar
1/2 cup cider vinegar
1/2 teaspoon salt
Blend tomatoes and purée from the can in a blender until smooth. In a heavy 4-quart saucepan, Cook onion in oil over moderate heat, stirring, until softened (about 8 minutes). Add puréed tomatoes, tomato paste, brown sugar, vinegar, and salt. Simmer, uncovered, stirring frequently, until very thick (about an hour). Purée mixture in a blender until smooth. Chill, covered, for two hours before serving.