Heat oil in a pan and temper with mustard seeds and asafoetida. After it stops spluttering, add the curry leaves, chana dal, urad dal and green chilies. Saute for a few seconds.
Add the semolina and stir fry for some time. Switch off the flame and transfer to a bowl. Add the yoghurt, sambar powder, coconut, salt, coriander leaves and required quantity of water to make a batter of pouring consistency. Set aside for 15 minutes.
Add more water if required. Divide the batter into 3 portions. In one portion add the beetroot puree and mix well. In the 2nd portion add the spinach puree and give it a stir. Let the 3rd portion remain as it is.
Now add a little of the eno fruit salt in each of the portions and give it a stir. Immediately spoon the batter in the greased idli moulds. Steam for 13-15 minutes and serve with coconut chutney and sambar.