Set a 1-quart saucepan over medium-high heat and add the oil.  Once the oil is hot, add the onions and garlic and sweat the onions, stirring occasionally, until translucent, 3 to 4 minutes.  Add the vinegar and cook until reduced by half, about 1 minute.  Add the remaining ingredients to the sauce pan and bring to a boil.  Reduce the heat to a simmer and cook until thickened to sauce consistency and flavors have come together, 4 to 6 minutes.  Remove from the heat and cool.  Sauce may be made up to 3 weeks in advance and refrigerated in a nonreactive airtight container.