Cover the washed fish pieces with some salt. Heat oil in a pan and fry the fish pieces slightly. Pick the fish pieces out and set aside.
Now add onion paste and fry in same hot oil for a bit. Add green chillies, salt, turmeric and red chilli powder powder, ginger and garlic paste and one cup water. Stir until the oil starts to simmer on top.
Add sliced jolpai (olives) to the curry and cook until the jolpai softens. Add the fish pieces and add another cup or two of water. Lower the heat and put lid on until the jolpai has boiled, the water has dried into a light gravy.
Pour the palm date jaggery ( khejur er gur) and stir well.
Serve with plain white rice or pulao.
Add the palm date jaggery only when the koromcha has been cooked, or else it will become sticky.
The dish is not supposed to be watery, but slightly gravy.
The amount of palm date jaggery used varies from person to person as it depends on how sweet one wants the dish to be.