Soak dal and rice separately for at least 4 hours. Grind them into a smooth batter adding just enough water. I usually grind them separately and then mix them together with a pinch of salt.
Set the batter in a warm place to ferment for at least 8 hours or until the batter is foamy and doubles in volume. In my kitchen, it takes about 18~24 hours to get to this stage.
Whisk ragi flour in 1cup of water to form a smooth batter with no lumps. Add this to the dal-rice mixture along with the curd (if using). Mix well.
Heat a tawa/ griddle on medium-high heat. Pour the batter on the tawa by the ladle.Spread it out with the back of the ladle to form a thin dosa. Cook till the dosa is crisp and golden on both sides. Drizzle some oil or ghee on the dosa while cooking. Serve hot with chutney. I served with some peanut chutney.