In a 3- to 4-quart pan over medium-high heat, stir onions in butter until limp, about 6 minutes.
Add tomatoes and their liquid, brown sugar, vinegar, pepper, allspice, cinnamon, and cloves; boil over high heat, stirring often, until mixture is reduced to 5 cups, 25 to 30 minutes. Add salt to taste.
Serve sauce hot, warm, or cool. Chill airtight up to 1 week.
Nutritional analysis per 1/4 cup.