Heat the oil in a large roasting pan over medium heat. Add onions to cook for 4 minutes, stirring frequently. Add sugar and the following 7 ingredients, cook for another 1 minute, stirring constantly.
Add vinegar, cook for 30 seconds. Add the broth and tomatoes, bring to a boil. Reduce heat and simmer for 55 minutes or until the sauce thickens, stirring occasionally. You can freeze such a sauce and store portioned cubes in the freezer for up to 6 months.