Combine all the ingredients in a nonreactive saucepan.

Place over medium heat and bring to a simmer.

Reduce the heat and simmer for 20 to 30 minutes, until the onion and garlic soften.

Transfer the mixture to a blender and blend until smooth.

Be cautious of hot liquid splattering.

Use immediately or allow to cool to room temperature before storing in an airtight container in the refrigerator for up to 4 weeks.