Soak rice and dhal together for 3 hours. Grind to thick, smooth paste. Add salt at the end of grinding. Allow to ferment for 18 to 20 hours. Next day morning grate coconut and take 2 extracts using 1 1/2 cups of water altogether for both extracts. Strain well and pour on top of the batter with sugar and soda. After one or 2 minutes mix it well. Heat a deep curved aappam pan and wipe it with a cloth which is dipped in oil. Pour a laddle full of batter in the centre of the hot pan. Lift pan from fire and tilt it in circular motion so that the aappam is formed. Centre of the aappam should be thicker than the sides. Close with a lid and cook in medium flame. Cook on one side only. When there is no uncooked batter, remove aapam carefully with flat laddle. Serve hot with onion chutney.