Marinate eggplant pieces with salt and hilsa pieces with turmeric and salt.
Soak cumin and turmeric powder with little water.
Heat mustard oil in a pan and fry hilsa pieces. Hilsa fish has a soft flesh so cooks very fast but if it contains egg as my one then fry golden brown on both sides at low flame. 3-4 mins should be enough to cook the fish eggs evenly.
Transfer fried fish pieces to a plate. Add eggplant to the remaining oil and fry at medium flame till soft and golden brown.
In the remaining oil add nigella seeds and green chillies. Saute for 15 secs.
Now add the soaked spices mixture and let simmer. When oil starts separating from spices add hot water and salt as required for stew.
Add fried fish and eggplant fishes to the stew and let simmer for 2 mins.