Rinse the rice and urad dal separately under running water. Soak the rice with fenugreek seeds in one bowl and the urad dal with poha in another bowl for at least 4-6 hours or overnight.
Drain the water from the rice and urad dal. Grind the urad dal and poha into a smooth paste using some water. Similarly, grind the rice and fenugreek seeds into a smooth paste. Mix both the pastes together in a large bowl.
Add salt to the batter and mix well. Cover and let it ferment in a warm place for 8-12 hours or overnight. The batter will rise and become slightly bubbly.
Heat oil in a pan over medium heat. Add mustard seeds and let them splutter. Add cumin seeds, urad dal, chana dal, and fry until golden brown.
Add the turmeric powder and mashed potatoes. Mix well and cook for a few minutes. Add salt to taste and chopped coriander leaves. Mix everything well and remove from heat. Set aside.
Heat a non-stick griddle or a cast-iron skillet over medium-high heat. Once hot, reduce the heat to medium.
Pour a ladleful of dosa batter onto the center of the griddle. Spread it out in a circular motion to form a thin dosa.
Drizzle some oil around the edges and cook until the dosa turns golden brown and crisp.
Place a portion of the potato masala in the center of the dosa. Fold the dosa over the filling and remove from the griddle.