Heat the oil in a large saucepan over medium heat. Sauté sliced onion in oil for 5 minutes, until onions are soft, but not brown. Add 3 cups water, tomato paste, corn syrup, corn starch and salt; stir. Bring to a boil, then reduce hear and simmer for 45 minutes. Add vinegar. Continue to simmer an additional 30-45 minutes or until most of the liquid has reduced and the sauce is thick.