Soak Urad dal atleast 6 hour or overnight and grind to paste. Mix rava (semolina), urad dal paste, yogurt and salt with water to make a thin batter. Leave in room temperature overnight. Mix onion and chilies to the thin batter. Heat pan or griddle with little ghee or oil. Spread the mix on pan in circular motion to make thin Dosa. * SEE TIP BELOW Cook on both the sides, if desired. Serve hot with Chutney.