Wash the parboiled rice, urad dal, and fenugreek seeds thoroughly. Soak them in enough water for at least 4 to 6 hours.

Drain the soaked ingredients and grind them together in a blender or wet grinder with fresh water until smooth and slightly fluffy. Stir in the salt and let the batter ferment in a warm place for 8 to 12 hours until doubled in volume.

Heat oil in a pan over medium heat. Add mustard seeds and let them splutter, then add curry leaves and sliced onions. Cook until the onions turn translucent. Stir in the turmeric powder and salt, add the mashed potatoes and water, mix well, and cook for 3 minutes before removing from heat.

Heat a tawa or cast-iron griddle over medium heat. Pour a ladleful of fermented batter in the center and spread it outward in a circular motion to form a thin crepe. Drizzle oil or ghee around the edges and cook until the bottom turns golden brown and crisp. Place a portion of the potato filling in the center, fold the dosa over, and serve hot.