Sacred Laundry

1 (28 oz) can whole tomatoes in puree (can use fresh tomatoes cooked and diced)
1 medium onion, chopped
2 tablespoons olive oil
1 tablespoon tomato paste
2/3 cup dark brown sugar (can substitute another sweetener, I would use 1/3 cup or less)
1/2 cup raw apple cider vinegar
1/2 teaspoon salt

Puree tomatoes 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 pureed tomatoes, tomato paste, brown sugar, vinegar, and salt and simmer, uncovered, stirring occasionally so it will not scorch, until very thick, about 1 hour.

Puree ketchup in blender until smooth.
Chill, covered, at least 2 hours (for flavors to develop).

Ketchup can be chilled up to 3 weeks.