Chop onions, tomatoes and coriander leaves finely and keep them aside till use. Slit green chillies and mix chickpea flour with 3 tsp of water and make the paste such that there are no lumps and keep it ready.
Wash potatoes under running water and pressure cook them till mushy. Peel the skin of cooked potatoes and crumble them once pressure subsides.
Method
In a wide cooking pan heat cooking oil, then add mustard seeds and lentils. As the mustard seeds pop up and lentils turn brown add curry leaves and chopped onions. Saute till the onion turn translucent. Then include chopped tomatoes, turmeric powder and slit green chillies.
When the tomatoes are cooked, include mashed potatoes and half cup of water. Stir and let it boil.
Finally include salt and chickpea flour paste. Add water if you find it thick. Let it cook in simmer for about 3 minutes. Include garam masla and stir in(if you are adding) Switch off the flame. Garnish with chopped coriander leaves.
Delicious, soupy rava idli, chapati side dish is ready to serve.
Notes
The potato sagu has to be little thinner in consistency when you are pairing it with rava idli. As a side dish with chapati thicker version of sagu is fine. So adjust water addition accordingly.
Do not skip adding chickpea flour mixture, as it renders unique taste to the sagu.