Marinate the fish with turmeric and salt, keep aside.
Whip the yoghurt, add the remaining turmeric powder, red chili powder and salt to the yoghurt, mix well, keep aside.
Dissolve the mustard paste well in about 3 tbsp of water, pour through a muslin cloth, keep the the mustard water aside. We shall need just the mustard water here.
Heat 3tbsp oil in a pan, shallow fry the fish, keep aside. If the fish is very fresh, there's no need to fry it.
To the same oil, add the kalonji seeds and green chillies, allow them to splutter. Lower the heat and add the whipped yoghurt while stirring continuously, cook for 2-3 minutes.
Add the mustard water, give it a hearty stir, throw in more green chillies if you like it hot.
Now gently place the fish in the gravy and bring to a gentle simmer. 2-3 minutes I would reckon.
Add the coconut paste, mix well and continue to cook over a low flame for another 3-4 odd minutes.
Adjust the seasonings and finish with a hearty drizzle of mustard oil.
Serve hot with rice.