Soak the rice and the dals together for at least 2 hours. Drain, and place in a blender along with all the other ingredients.
Add enough water to keep the blades moving and grind until you have a fine batter. It should be just a little grainy so the adai turns out crispy, but not too coarse. The batter should be spreadable but thick enough to coat the back of a spoon.
Heat a griddle, either cast-iron or non-stick.
Using a ladle with a rounded bottom, scoop up 1/2 cup of the Adai batter.
Pour the batter into the center of the hot griddle and, quickly, spread it outward in concentric circles using the bottom of the ladle. Work fast, and don't panic if it doesn't look perfect the first time. No one gets it right in the beginning.
Pour a few drops of oil around the edges of the Adai and on top so it turns extra-crispy.
When the bottom of the Adai looks crisp and golden, about 1-2 minutes, flip over and cook the other side for another minute.
Serve hot with chutney and sambar.