Strain the water from the soaked urad dal and keep it aside. Now add the soaked urad dal and fenugreek seeds to a grinder, along with the strained water, and grind them to a light and fluffy batter. Add more water if required. Add the lentil batter to a bowl and repeat the same process with the soaked rice and poha, but use only fresh water (not strained water). Grind the rice and poha with 1 cup of water into a smooth consistency. 

Add the rice batter to the same bowl as the lentil batter. Add a little salt and mix it all together. Make sure both the batters mix well. Cover the batter and let it ferment for 8 to 9 hours. The fermented batter should have small air pockets. 

In a pan, add some oil. Lower the flame and add mustard seeds. Once they start spluttering, add curry leaves, onions, green chillies, and ginger. Sauté till the leaves turn crisp, and the onions turn translucent. Add the turmeric powder and ¼ cup of water.

Add the boiled and chopped potatoes and mix them well with the masala base. Mash the potatoes, season with salt, and let the dish simmer on a low flame for 3-4 minutes till the water dries. The potato filling should be moist enough to spread easily but not have a curry/gravy-like consistency.

Heat a pan on medium flame. Smear a little oil on the pan and lightly stir the batter. Now, pour a ladleful of batter on the hot tawa. Spread the batter in a circular shape. Drizzle very little oil on top and edges to cook the dosa. Once the base cooks (turns golden brown), add a small amount of spiced filling in the centre and spread. Gently fold the dosa and serve it hot. Each dosa takes 3-4 minutes to cook.