In a small saucepan, bring the rhubarb and water to a simmer and cook for 5-6 minutes, until the rhubarb is very soft. Pour into a bowl and set aside.
In the same saucepan, heat a drizzle of oil over medium-high heat and cook the onion for 3-4 minutes, until soft; add the garlic and cook for another minute or two. Add the remaining ingredients and bring to a simmer; cook for 10-15 minutes, until the mixture thickens. Puree with a hand-held immersion blender or cool and puree in a blender or food processor until smooth. Use as you would any barbecue sauce - on grilled meats, in baked beans or drizzled on burgers.
Serving Size Makes about 2 cups.