Mix the rice and dals together and wash well. Soak them in water for 3 hours. Grind soaked rice, dals, red chilies, sweet corn and required salt in a mixer jar to coarse batter adding water. The batter should be in pouring consistency.
Add the chopped onions and curry leaves, Can add half tea spoon of hing / asafoetida powder if you like the flavor. If adding onions, put them in to to the blender when the batter is ready and give one or two turns and this helps to make smooth, thin and crispy adai or dosa.
Heat a dosa tawa, grease it with half tea spoon of oil and pour a ladle full of batter on the tawa once it is well heated and spread to thin dosas
Make a small hole in the centre and drizzle some oil in the centre and around. After a minute, flip it off to the other side. Once both the sides are crispy well cooked the adai is ready to serve.