In a small saucepan, bring rhubarb and water to a boil. Reduce heat; simmer, uncovered, or until tender, 5-6 minutes. Remove from heat; cool slightly.
Place rhubarb in a blender or food processor; cover and process until smooth. Set aside.
In the same saucepan, cook onion in oil until tender. Add garlic; cook and stir 1 minute longer. Add the remaining ingredients.
Whisk in rhubarb puree until blended. Bring to a boil. Reduce heat; simmer, uncovered, for 5 minutes. Use as a basting sauce for grilled meats. Store in the refrigerator.