Fermentation time: 6-10 hours. Please try to follow the water measurements if you are not familiar with the consistency of the dosa batter. It makes a lot of difference.
Wash and soak urad daal and methi seeds in 3 cups of water for 4 to 6 hours.
Drain the soaked urad daal and without rinsing add this to mixer or food processor. Grind using a little water as needed to make a smooth batter. (I used 1 and 1/2 cups of water for grinding daal)
Add rice flour into a big bowl and add water as needed and mix well with your hand, whisk or spoon so that no lumps remain. Add the ground daal and remaining water if any.
Let it ferment for 8-10 hours. You know it's fermented when you see tiny bubbles and when the batter increases in volume.
Add salt to the batter. Heat a flat griddle / Tava on medium-high heat. Using a flat ladle, pour the batter in the center of the griddle and flatten it from the center towards outside in concentric circles.
Add oil, butter or ghee(clarified butter); ghee is anytime better. Wait for it to cook and turn crisp. Once the desired crispiness is achieved, fold it in half or roll it and place it on a plate. Don't cook the other side since that will make it soft.
If the batter is too thick and you have difficulty spreading it, add some more water.
Fermentation depends on how warmer the room is. I store my batter in my turned off oven or microwave. Let the oven light stay on for an hour.
The longer the batter stays outside, it will become sour. If you don't plan on making dosa, transfer the dosa batter into the refrigerator.
If you are in a hurry, you can soak the urad daal in warm water and grind it within an hour or two.
I ideally soak daal in the morning and grind in the evening so that batter is ready by morning, or else I soak daal in the evening and grind in the morning so that batter is ready by evening. Time management I say :)
Once the batter is fermented, you can also freeze it. To thaw, transfer into the refrigerator 6-8 hours before you want to make dosa.
It is also ideal to let the batter stay at room temperature at least 30 minutes before you start making it.
Pan shouldn't be too hot when spreading the batter. If it gets hot, sprinkle some water before spreading the batter.
Batter stays good for 3-4 days if stored in the refrigerator.
P.S: How you spread the batter is important to achieve crispiness. Please check out any of the good videos on youtube.