Marinate the fish with turmeric and salt. Heat mustard oil in frying pan and fry the fish to golden brown. Keep the fish aside.
In a bowl, lightly mix the mustard paste, whipped curd, tomato puree, turmeric powder, red chilli powder, salt and sugar.
Reheat the same oil (as was used to fry the fish), lower the heat and slowly add the above mixture. Allow the mixture to come to a gentle simmer.
Add the fried fish slices. Cover and cook on low heat till the fish is tender.
Now add the coconut milk slowly while stirring continuously. Add the green chilies and salt if required.
Continue cooking on a low flame till the gravy thickens. Serve hot.