In a large bowl, mix all the flours with salt and add water slowly to get a smooth batter.
Let the batter soak for 2 hours.
Heat oil in a small pan for tempering and add the mustard seeds.
When it splutters, add the onions, green chilies, chopped cashew nuts and fry over low flame till the onions turn soft and pink.
Pour this into the batter. Add finely chopped coriander leaves and curry leaves.
Adjust the water quantity in the batter. We will need a batter with pouring consistency.
Heat a tawa over a low flame and grease it uniformly with oil. If you are using a nonstick pan, ensure you wipe it clean.
When the tawa is hot, pour the batter using a ladle and swirl for the batter to spread evenly.
Drizzle oil and cook over low flame until the dosa leaves the sides. Flip to cook the other side.
Repeat the process for all the dosas.
Serve with coconut chutney and sambar.