Heat oil in a pan, shallow fry the fish (I can see your frown there (smiley) yes, it might be blasphemy to some who doggedly persist that you should never fry the fish. Take a call – if the ilish is indeed very fresh, you need not fry it).
Marinate the fried fish with rest of the turmeric, red chilli powder, whipped curd and just a pinch of salt.
To the same pan, add the rest of the oil, when smoking hot, throw in the nigella seeds and green chillies.
When they stop spluttering, turn the heat to low and gently slide in the fish along with the marinade. Cook for a couple of minutes till the curd starts to release oil.
Now add a little warm water to the mustard paste, add to the yoghurt gravy and simmer for 4-5 minutes.