Put olive oil in a medium saucepan over medium heat. Add the diced onions and saute for 5 minutes.Add the rest of the ingredients and, stirring frequently, c cooking for about 15 minutes. Taste for seasoning. You might want to add more salt, vinegar or brown sugar, to taste.
If you want a 'smoother' sauce, cook for another 10 minutes. Remove from heat. Let cool, then put into a covered container in the refrigerator. This will keep for about a week.