Transfer the dals, rice, chillies, and methi seeds to a bowl and cover with water. Set aside for a minimum of 5 hours. .
Once the dals are fully soaked, drain the water and set aside. Reserve the soaking liquid.
Transfer the soaked dal and rice mix to a high powered blender and make a smooth paste using a little bit of the leftover water as needed. It is better to add a little water at a time, since there is no easy way to fix a batter that is too thin.
Empty the contents of the blender to another large bowl or other container such as a dutch oven.
Add salt to the batter and mix well. Dilute the batter with water until you have a pourable batter. Cover and ferment the batter for up to 24 hours in a warm place.
Once the batter has fermented and has increased in volume, stir it back down and transfer to an airtight container and refrigerate until ready to make the adai.
The batter can be stored in the refrigerator for up to a week, but it is best used within 2-3 days.