Mix the rice and black gram and soak it in water for 5-6 hours. Drain and grind it to a coarse batter. Keep the batter aside to ferment for 2-3 hours.
Mix the chopped green chillies, chopped coriander leaves, chopped onions and salt to the batter. Heat a tawa and grease it with oil. Pour a spoonful of batter in the middle of the tawa and spread evenly.
Cook the uthappa till crisp on both the sides using oil when required. Serve hot