Wash the hilsa pieces and rub them with 1 teaspoon of turmeric powder and 1 teaspoon of salt. Set aside. You can fry the pieces at this stage or add them raw to the gravy later.

Grind the remaining 3 onions, ginger, and garlic into a paste. Heat the reserved mustard oil, add the onion paste, and fry for 3-4 minutes until golden brown. Add the ginger-garlic paste and cook for another 3-4 minutes. Stir in the remaining salt and Kashmiri chilli powder, then add the beaten curd and cook until the oil visibly separates from the mixture.

Add the hilsa pieces to the pan, cover, and cook over low heat for 5-7 minutes. Open the lid and turn the fish, making sure to keep the pieces intact. If the gravy looks like it's drying out, add 1-2 tablespoons of water and simmer another 5-7 minutes for a thick, non-runny gravy.

Wash the rice 3-4 times and soak it in water for 30-40 minutes, then drain. Bring double the volume of water to a boil in a deep, thick-bottomed pan with a few whole cardamom pods, a bay leaf, cloves, and cinnamon. Add the rice and salt, cover, and cook on medium heat for about 10 minutes, until the rice is roughly 90% done. Turn off the heat and let it stand, covered, for 5 minutes.

Heat the ghee in a pan and temper it with the remaining bay leaf, dry red chillies, cinnamon, and cardamom. Mix this carefully through the parboiled rice.

Spread half the rice in the pan. Arrange the hilsa pieces on top and spoon the thickened gravy over them. Scatter half the birista and half the slit green chillies over this layer. Add the remaining rice on top, followed by the rest of the birista and green chillies. Cover and cook on low heat for 5-7 minutes, then switch off the flame and let the pulao rest, covered, for another 5 minutes before opening.