Place all of the ingredients in the blender and blend until smooth. Transfer the mixture to a saucepan and cook it over medium heat for 5-10 minutes, stirring occasionally. If the sauce gets too thick, add a little more water. The cooking step is to remove the raw taste of the shallot and the rhubarb, so taste it after five minutes to see if you think it needs more time.
Remove the sauce from the heat and taste to adjust the seasonings, if needed. This makes about two cups of sauce.