1. Preheat oven to 300°. Place tomato halves in a colander, and press gently to remove excess liquid.
2. Stir together tomatoes, onion, and remaining ingredients in a large bowl. Place tomato mixture in a single layer in an aluminum foil-lined 15- x 10-inch jelly-roll pan.
3. Bake at 300° for 2 1/2 to 3 hours or until tomato mixture is deep red and lightly browned, stirring every 45 minutes. Cool in pan on a wire rack 15 to 20 minutes or until completely cool. Store in an airtight container in refrigerator up to 3 days.
To freeze: Place tomato mixture in an airtight container, and freeze up to 3 months. Thaw in refrigerator 24 hours.