Soak rice for 20 minutes.

Heat 3 tablespoons ghee in a kadai.

Fry chopped onions until translucent and pale golden.

Add ginger garlic paste and red chilli powder, fry until the mixture comes together, sprinkling water if needed (koshano/bhuna).

Add whisked mishti doi and salt, stir well to mix.

Pour half of the coconut milk and stir quickly.

Add raw fish pieces carefully in a single layer.

Add green chili and cook for 4-5 minutes, depending on fish size.

Flip fish pieces gently.

Add rest of the coconut milk and cook for 10 minutes with cover or until fish is cooked thoroughly.

Keep the fish covered until needed.

Heat 1 tablespoon ghee in a cast iron biryani pot.

Add whole garam masala to the hot oil.

Add half of the gravy from hilsa along with luke warm water, bring to a boil.

Add previously soaked rice and let it cook for 10-12 minutes or until rice is almost half cooked.

Add coconut milk, salt and cook covered for another 10 minutes or till the rice is completely cooked.

Arrange the ilish pieces on bed of rice and pour the gravy on it.

Add green chillies.

Cover and cook for 5-7 minutes on simmer.

Remove from flame but keep covered until ready to serve.