Ilish Shorshe Posto Or Hilsa in a mustard and poppy seed curry

I was pleasantly surprised to see the 'Queen of Fishes', the hilsa, at my local fish monger. With my older daughter coming home for the weekend, I eagerly picked one up. She does crave for Bengali food at her hostel, so the hilsa would be the star in the Bengali meal I would be preparing.
This is my late mother in law's recipe.

1 kg hilsa, sliced and marinated in salt and turmeric pd
2 tbsp of yellow mustard seeds
3 tbsp of poppy seeds
4-5 green chilies
1/2 cup fresh coconut
3-4 tbsp mustard oil
1 tsp panch foron
1 tsp turmeric pd
1 tsp chili pd
2 green chilies, slit for garnish
Salt

Grind the mustard seeds, poppy seeds, green chilies and coconut to a fine paste with enough water.
Heat oil in a pan, lightly fry the fish slices and keep aside.
Heat the remaining oil and add the panch foron. Now add the ground paste, salt, turmeric and chili pd. Saute for a couple of minutes. Now gently place the fried fish in this curry. Add water to make a thick gravy. Add the slit chilies and simmer for 5-6  minutes.  Garnish with a drizzle of mustard oil and serve with white rice.