In a small saucepan set over medium-high heat, saute the onions in olive oil for 8 minutes. Add the garlic & cook another 2 minutes, or until onions are very soft & golden. Stir in the remaining ingredients. Bring the mixture to a simmer, then reduce the heat to low & cook, stirring occasionally, for 20 minutes for the flavors to marry.

Using an immersion blender or food processor, puree the mixture until very smooth. If the sauce is too thick or chunky, add a little water (a tablespoon or two at a time), until the desired consistency has been reached.