Wash and soak the millets and urad dal separately for about 1-2 hours

In a mixie, first grind the urad dal to a smooth paste using enough water

Set aside in a large bowl that can hold the entire batter

Then, grind the varagu arisi to a smooth paste with water

Add to the same bowl as the urad dal

Add salt and mix thoroughly to combine

Leave covered in a warm place (see notes if you live in a cooler place) for about 8-10 hours for the batter to ferment

After about 6 hours, my batter had fermented with nice bubbles showing on the surface of the batter. I left it out for another hour or so and then refrigerated the batter

Heat a dosa tawa until hot. Grease with some oil

Take about ¼ cup batter and make into thin dosa

Add more oil on top as the dosa cooks

Flip when the first side is a light brown

Cook the second side until done

Serve hot with sambar and/or chutney