Heat oil in a large frying pan over medium-high heat. Add mustard seeds, cumin seeds, chana dal and urad dal. Sauté for few minutes or until the mustard seeds splutter and dal turn golden.

Add asafetida and ginger. Sauté for a few seconds. Add onions, green chilies and curry leaves. Sauté for 3-4 minutes or until the onions turn golden brown. Add ground turmeric and sauté for a few seconds.

Add potatoes. Season with salt. Pour in water. Cover and cook, stirring occasionally, for 5 minutes or until the potatoes are tender. Use a fork to lightly mash the potatoes. Stir in coriander (cilantro) leaves. Remove from heat. Set aside.

Place rice in a bowl and cover with cold water. Set aside. Place urad dal and fenugreek seeds in another bowl and cover with cold water. Set aside. Place poha in a small bowl and cover with cold water. Set aside. Soak for 4-6 hours. Drain. Set aside.

Place rice in the bowl of a food processor with 1 cup water. Process to a smooth batter. Pour this into a large bowl. Place urad dal and fenugreek seeds into the food processor. Add 1 cup water. Process to a smooth batter. Pour into the bowl with the rice mixture. Place poha in the food processor with ½ cup water. Process to a smooth batter. Pour into the bowl with the rice and dal mixture. Stir to combine. Cover and place the bowl in a warm spot for about 8-10 hours. Leave to ferment.

Season the fermented batter with salt. Stir.

Heat a tawa or pan over medium-high heat. Use a cut onion or paper towel to lightly grease the tawa with oil. Pour a ladleful of dosa batter onto tawa. Use the back of the ladle or spoon to spread the batter thinly onto the tawa in a circular motion. Pour a little oil on top of the dosa. Cook. When the bottom of the dosa starts to turn golden, add the potato masala, fold, and remove from tawa. Serve hot.