Wash and soak the rice and lentils for a couple of hours. After soaking, drain the water. Add the rest of the ingredients and grind to a smooth batter. Add up to a cup of water while grinding. Depending on the age of the rice, you may require a little bit more water or less. If the batter is very thick, do dilute the batter with little water.
Once the batter is ground and ready, you can use it immediately. If not using immediately, store it in the refrigerator and use it within a day. This batter does not last longer than a day. So, use it within a day of making the batter.
To make dosai:
Heat a dosa griddle until hot on medium flame. Ladle half a cup of batter and spread it on the pan. Sprinkle 1/4 teaspoon of oil. Flip the dosai once after about a minute. Cook the dosai until golden on both sides.
Serve hot with coconut chutney.