Preparation / Directions:
Lightly wash urad, and cover with chlorine-free water. Grind methi and mix with urad, along with salt, and allow to soak uncovered for 3 hours. Soften the cream of rice by soaking for the final 15 minutes. Without washing, grind urad in food processor to a fine paste. Add cream of rice to paste, and grind again until mixed well. Place in ceramic bowl, cover with plastic wrap, and place in a warm place until double in bulk. The amount of time this will take depends on the temperature. The batter will double in 8 hours if the temperature is over 85F, and at lower temperatures it can take as long as 30 hours. If the batter doesn't rise, don't despair, just find a warmer place. A small microwave oven, with the door open just enough to turn on the light, works well, as does a cold oven with the light on.
Many silly people substitute rava (sooji, cream of wheat) for rice, especially in colder climates like the Frisco Bay Area, because their batter doesn't rise. The fermentation process depends on the presence of wild yeast, which appears to stick well to the urad and to the methi. Soaking the dal uncovered allows you to "catch" additional yeast from the air. Don't use baking soda, baking powder, yeast, or yogurt to "help" fermentation. I've conducted controlled experiments on these agents, and methi out-performs them. Yeast starts fast, but it poops out and leaves your idli tasting like elephant cacoos. Baking soda, baking powder, and yogurt retard fermentation.
After your batter has gone nuts, you are ready to make idlis. Stir (don't beat) the batter slightly, just enough to evenly distribute the bubbles and not enough to make it subside. Grease idli cups (you can use egg poachers if you want) with spray-on oil or butter, and fill them (almost full) with the idli batter and steam in an idli vessel (basically, a big steamer) for about 15-20minutes.
The same idli batter can be use to make Sannan. Pour the batter in the container of a pressure cooker (previously greased with oil) about 3/4 full and steam for 15-20 minutes as above. Take out and cut into pieces with a spatula.
Source: Richard B