Active time: 20 min Start to finish: 3 hr (includes chilling)
Makes about 2 cups.
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
Purée tomatoes (with purée from can) in a blender until smooth.
Cook onion in oil in a 4-quart heavy saucepan over moderate heat, stirring, until softened, about 8 minutes.
Add puréed tomatoes, tomato paste, brown sugar, vinegar, and salt and simmer, uncovered, stirring occasionally, until very thick, about 1 hour (stir more frequently toward end of cooking to prevent scorching).
Purée ketchup in 2 batches in blender until smooth (use caution when blending hot liquids).