Take urad dal /split black gram,fenugreek seeds, and split bengal gram / chana dal in a bowl.
Wash and add 1 cup of water. Soak it for 3 to 4 hours.
In a separate bowl take oats and poha/avalakki. Wash and add around 1.5 to 1 ¾ cup of water and soak it for 3 to 4 hours overnight .
Transfer some water from the soaked dal, grind it to a fine paste. Add this water if needed to grind. Transfer it to a big bowl
Now in the same jar grind the oats to a fine paste. always transfer some water into a cup and then grind. add more water if needed to get the right consistency of the batter
Add this to the urad dal batter, mox it well. The consistency of the batter should be little thick ( like set dosa batter) compared to regular dosa batter,
Cover and keep it aside for 6 to 8 hours in a warm place to ferment.
To the fermented batter add 1.5 tsp or salt to taste, mix it well. Now the batter is ready to make Dosa.