Heat oil in a cooking vessel, add mustard seeds and let it splutter. Add urad dal and channa dal and fry till they turn red. Add fresh curry leaves, green chillis and ginger and saute for a few secs.
Add the sliced onions and saute on medium heat for 4-5 mts or till the onions turn transparent. Add the turmeric pwd and combine. Add the crumbled boiled potatoes and adjust salt. Place lid and cook for 3 mts.
Add a cup of water to the potatoes and increase flame, cook for 2 mts. Reduce flame and place lid and cook for 4-5 mts.
Remove lid, sprinkle the besan all over the cooking potatoes and give a good stir. Simmer till the rawness of besan/chick pea flour disappears, approx 3-4 mts and the curry slightly thickens.
Garnish with coriander leaves and serve with puris.