Heat 2 tbsp oil in a pan and add mustard seeds when they start spluttering add cumin seeds and in 5 secs add green chillies and chopped onions
When onions start getting brown add turmeric and let it cook for about a minute
Now add potatoes and mix them well
Add one cup of water and let it cook nice and moist like this . Once it's completely done add cilantro and son line juice (optional) and you are done :)
Add this as a stuffing in the dosa or have it as a side with some naan. Enjoy!!!