Put all the ingredients in a blender or a dosa grinder and blend with enough water till you have a runny and smooth batter. The batter should be runnier than a pancake batter, but thick enough to coat the back of a spoon.
Heat a cast-iron or non-stick griddle.
Using a rounded ladle, pour about 1/4 cup of the dosa batter in the centre of the griddle.
Quickly, using the rounded bottom of the ladle, start spreading the batter outward from the centre in a spiralling design, until you have a dosa about 6-7 inches in diameter.
You can pour a few drops of oil around the dosa's edge to make it crispier, or just spray with some non-stick cooking spray.
When the underside is golden-brown, flip over and cook another minute or two.