Grind the onions, coconut, ginger, garlic, green chilies, chili powder, coriander powder, cumin powder & a pinch of turmeric powder with little water. Whisk the yoghurt and keep it aside. Boil the potatoes with salt & turmeric powder. When almost done, remove from heat. Drain the water.
Heat oil in a pan and add mustard seeds. When they starts to pop add the ground mixture and fry for 10 minutes. Then add the yoghurt and mix well. Add potatoes and 1/2 cup of water. Lower the heat and cook until done. Add little salt if necessary. Garnish with chopped coriander leaves.