In a large saucepan, heat oil over medium heat. Sauté onion until soft and translucent, about 10 minutes.
Whisk remaining ingredients into the saucepan. Bring mixture to a boil, and turn down heat to medium-low. Simmer for 30-45 minutes, stirring occasionally.
Remove from heat. Blend sauce together using an immersion blender (or regular blender but be careful), making sure that all of the onion is blended into the sauce. Whisk sauce together. Return saucepan to medium-low heat, and cook an additional 10 minutes.
Taste sauce, and adjust seasonings as needed. Allow the sauce to cool slightly before storing it in an airtight container in the refrigerator. The sauce will last up to 2 weeks in a refrigerator and about 1-2 months frozen. Enjoy!