Blend soaked rice and urad dal with water until smooth. Ferment for 8 hours or overnight.

Heat oil in a pan, add mustard seeds, and let them splutter. Add onions, green chilies, and sauté until onions are translucent. Add turmeric and mashed potatoes, mix well, and season with salt.

Heat a non-stick skillet, pour a ladle of batter, and spread it into a thin circle. Cook until the edges lift, place some potato filling in the center, fold, and serve hot.