Parboil the potatoes with salt to taste. Drain and keep aside. Heat oil in a pan and season with mustard seeds. When they splutter, add urad dal and stir until light golden. Add onions and sauté until translucent and glossy. Add ginger and green chillies and stir for a few seconds. Add curry leaves and stir well. Add spinach and stir-fry till almost dry. Add potatoes and stir-fry till done and dry. Add the grated coconut and stir for a minute. Remove from heat and adjust the seasoning