Soak the rice and lentil in water separately for 5-6 hours.
Grind the lentil to a fine smooth paste, and the rice a little coarse (enough to feel a texture, like fine sand).
Mix the two ground ingredients with salt and enough water to form a batter of dropping consistency.
Leave, covered, in a warm place to ferment to a spongy batter, overnight.
Steam in an idli steamer and serve hot.