In a jug add buttermilk and gram flour.add water if needed.now blend it with blender.in a kadhai add oil.now add cumin seeds and carom seeds.now add green chilli and onion.cook for two minutes.now add potatoes and cook for five minutes.add all spices and mix well.now add buttermilk mixture and stir countinously till it boils.after that cook it for ten minutes.now add coriander leaves and serve hot with rice.