Blend all the ingredients in a high-powered blender, adding enough water to make a smooth batter, with a consistency that falls right between a crepe batter and a pancake batter.
Heat a non-stick or well-seasoned cast iron griddle.
When the griddle is hot enough that a drop of water skitters off and evaporates immediately, pour the batter (about ½ cup) in the center of the griddle, using a round-bottomed ladle.
Using the bottom of the ladle, start spreading the dosa, moving from the center and outward in a spiraling motion. You want a fairly thin crepe. Don't worry too much if you mess it up before you get it right. No one mastered a dosa the first time they made it. No one.
If you wish, spray some oil on the dosa and its sides. This is not absolutely essential if you're using a non-stick pan, but it's advisable if you want crispy edges.
Let the dosa cook until the underside is golden-brown. Flip over and cook for a few seconds.