Boil potato and peel the skin. Chop onions vertically and slit chilies also vertically. Cut the potato into small pieces. In a Pan heat oil and add mustard seeds, cumin seeds, curry leaves, green chilies. When they start to splitter add onion and fry till they are transparent. Add turmeric powder, salt and water and let it come to boil. Add besan flour evenly and stir to avoid lumps and cook till you get the desired consistency. Serve hot with poori.