Pre-heat oven to 400°F.
Cut tomatoes in half and place cut-side up on a rimmed baking sheet.
In a bowl, mix together the garlic, onions, salt, pepper, sugar, Italian seasoning, basil, and vinegar.
Pour the oil mixture over the tomatoes and make sure they are all covered.
Place the baking sheet in the oven and roast for 30 minutes. After 30 minutes, turn the tomatoes. Roast for an additional 15 minutes. Tomatoes should be soft and juicy.
Remove tomatoes from the oven and allow them to cool for 15 minutes.
Place in a food processor. Process till desired consistency.
This sauce will keep fresh in the refrigerator for about 5 days, or freeze for up to 6 months.