Blend soaked rice, urad dal, fenugreek seeds, and water until smooth. Transfer to a bowl, add salt, and let it ferment overnight.

Heat oil in a pan, add mustard seeds and let them splutter. Add onions and green chilies, sauté until onions are translucent. Add turmeric and mashed potatoes, mix well. Stir in coriander leaves and cook for 2-3 minutes.

Heat a non-stick skillet, pour a ladle of batter and spread it in a circular motion. Drizzle oil around the edges. Cook until the bottom is golden brown, place some potato filling in the center, fold the dosa, and serve hot.