In a small saucepan simmer the rhubarb with the water until the rhubarb is very soft.
In a medium pot saute the onions in the oil until soft but not brown. Add the garlic and cook another minute.
Add the rhubarb mixture along with the remaining ingredients and bring to a simmer; cook for 10-15 minutes, until the mixture thickens.
Use an immersion hand blender, or transfer sauce to a blender to puree until smooth.
Store in the refrigerator