Place water, lemon juice, coconut, cilantro, ginger, green chili, cumin and salt in the order given in the container of a blender. Blend until reduced to a thick puree, adding a little more water if necessary. Pour into a small bowl.
Heat oil in a 6 inch / 15 cm skillet over medium low heat. Add black mustard seeds. When the seeds start crackling, remove from heat. add to the contents of the bowl.