Wash and soak the dals and rice for 4- 5 hours. Grind to a fine paste and let to rest for 1 hour.
Soak Tamarind separately in a bowl till soft and extract pulp.
Grind Onions, Red chilis, coconut, coriander leaves to a fine paste.
Then mix this onion paste, tamarind pulp, salt to the ground batter.
Heat a Tawa or pan with oil, pour one ladleful of batter and spread as a dosa. Sprinkle oil on top.
Cook on one side and flip on the other side.
Notes
The batter should be thin and cooked on medium flame so that the dosa comes out crispy.The batter can be soaked for longer hours too but after grinding you can store it in the fridge.Red chilies can be increased as per taste. I have used only about 50 gms of all, yet after soaking and grinding I had more batter than I thought.So adjust accordingly to your requirement keeping in mind the proportions.