Put the cucumber, water, garlic, and flax seed into a blender or food processor and blend on high speed until frothy. Add the remaining ingredients (except sumac) and blend on high until smooth. Pour into a bowl and refrigerate until chilled and slightly thicker. Sprinkle with sumac and serve.
Variations: Add one or more of the following to taste:
chipotle chile powder
Sriracha ("rooster") sauce or other hot sauce to taste