Soak rice and lentils separately in water for 4-6 hours or overnight. Drain both.|2. Grind soaked lentils with a little water to form a smooth, fluffy batter. Transfer to a large bowl.|3. Grind soaked rice with water to form a smooth batter, slightly coarser than lentil batter.|4. Combine both batters, add salt, and mix well. Cover and ferment in a warm place for 8-12 hours until doubled in volume.|5. For potato masala: Heat oil in a pan, add mustard seeds and let them splutter. Add curry leaves, green chilies, and ginger.|6. Add turmeric and diced onions, sauté until golden. Add boiled mashed potatoes and salt, mix well. Set aside.|7. For coconut chutney: Blend coconut, green chilies, ginger, and salt with water to form a smooth paste.|8. Temper with mustard seeds and curry leaves in hot oil, pour over chutney.|9. Heat a flat griddle or crepe pan on medium-high heat. Lightly grease with oil.|10. Pour a ladleful of batter in the center, quickly spread in circular motions to form a thin crepe.|11. Drizzle oil around edges, cook until bottom is golden and crisp (2-3 minutes).|12. Place a portion of potato masala in the center, fold dosa in half.|13. Cook for another minute, then transfer to plate. Serve hot with coconut chutney.