Wash urad dal, chana dal, and methi seeds together in a large bowl. Soak them for four to five hours.
Wash and soak rice in another large bowl and let that sit for four to five hours as well.
30 minutes before you start making the batter, rinse poha twice through fresh water and let is soak for the 30 minutes.
Drain the water from the dal mix completely and add it to the blender.
Pour 1 ¼ cup water and salt.
Blend it into a thick, frothy, and smooth paste. Transfer it into a bowl.
Drain the rice and transfer it to the blender.
Add ½ cup of water to it and blend the rice into a slightly coarse or smooth paste as per your liking. You can add more water to balance the consistency.
Transfer the rice batter into the dal batter and mix them well together.
Let the batter ferment in a warm place for 6 to 16 hours depending on the weather. (The higher the temperature the fewer the hours).
If you live in a cold place then you can keep it in an oven which is preheated at 60 degree Celsius for 10 minutes. Leave the oven light on.
Start preparing the potato masala when the batter is ready.
Boil the potatoes until they are just done. Don’t boil them until they are mushy.
While the potatoes are boiling, soak chana dal and urad dal in hot water to soften them. After 30 minutes, drain the water and keep the dal aside.
Heat oil in a pan. Add cumin, mustard, chana dal, and urad dal and saute them until the dals turn golden.
Add onions, curry leaves, and chillies and saute till the onions turn lightly golden.
Add hing and ginger paste to the pan and fry it until you can smell the aroma of cooked ginger.
Crumble the potatoes into the pan. Add turmeric, salt and 2-3 tablespoons of water only if the potatoes are firm and not mushy.
Saute the mixture for two to three minutes before adding coriander leaves. Taste and adjust the salt as per requirement.
Now before you start to make the dosa check the consistency of the batter. It should be thick, pouring, and also should spread smoothly. Add water if needed.
Heat a dosa tawa or cast iron pan at medium heat. Season the cast iron pan with oil before you use it.
In case you’re using a cast iron pan, sprinle a little water in to check if its hot enough.
Stir the batter once and scoop a ladle full of it.
Pour it at the center of the hot tawa and start spreading it immediately in circular motion. Keep making outward circles until you reach the edge of the pan.
You can adjust the thickness of the dosa as per your requirement.
If you can’t spread the batter then either the batter is too thick or the pan is too hot.
Pour half a teaspoon of batter around the edges of the dosa.
Flip it around and cook it just for a minute.
Flip it back and cook until the base is goden and crispy.
Spread the potato masala on one side of the dosa and fold it.
Keep repeating the process for as many dosas as you want.
Serve them hot with coconut chutney and sambhar.