Soak the rice and all the dals in lukewarm water for 2-3 hours. Grind red chillies, cumin seeds, curry leaves, peppercorns and coconut with a little water to a paste.
Add all the dals and rice and grind a little coarsely with a little water. Transfer the batter into a bowl. Add asafoetida and salt and mix well.
Add sufficient water to get the correct consistency and mix. Add onions and mix. Heat a non-stick tawa and drizzle a little oil on it.
Rotate the tawa so that the oil spreads all over the surface. Drop a ladleful of the batter and spread to a round dosa.
Make a small hole in the centre and drizzle a little oil into the hole. Cover and cook for 1 minute. Flip, drizzle some oil in the hole and cook for 1 minute more or till the adai is crisp and golden on both the sides.
Serve hot with aviyal. It can also be served with grated jaggery and fresh home made butter.