Transfer the urad dal, poha, and methi seeds to a bowl and cover with water. Set aside for a minimum of 5 hours.
Once the urad dal is fully soaked, drain the water from the dal. Reserve this extra soaking liquid.
Transfer the urad dal to a high powered blender and make a smooth, fluffy paste using water as needed. It is important to only add a little water at a time to get a nice fluffy texture. Use the reserved water from soaking the dal first. Remember that you can always dilute the batter later if it is too thick, but cannot fix a batter that is too thin easily.
Empty the contents of the blender to another large bowl or other container such as a dutch oven.
Add the rice flour to the urad dal paste.
Add salt to the batter and mix well. Dilute the batter with water until you have a pourable batter.
Check the salt by tasting a drop of the batter. You should be able to tell that there is salt in the 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 dosas.
The batter can be stored in the fridge for up to a week, but it is best used within 2-3 days.