Put some oil into a pan, allow to heat. Add cummin and mustard seeds, let them start to sizzle then add the onions. Wait until they are transparent, add the garlic & ginger, add the chopped tomato, tumeric, green chillies and salt. Allow to cook for 3 mins, add hot water and let simmer for another 3 or 4 mins. Add potatoes and hot water according to the consistency you like. Finish off with coriander