Soak the poppy seeds in warm water for about 15 minutes. Drain and grind into a smooth paste with 2–3 green chilies.
Lightly rub the fish pieces with salt and a pinch of turmeric. Let them rest for a few minutes.
Heat mustard oil until it reaches its smoking point, then reduce the heat. Shallow fry the fish pieces lightly until just golden. Remove and keep aside.
In the same oil, add the posto paste. Cook on low heat for 3-4 minutes.
Add the remaining turmeric powder. Then gently stir in the whisked yogurt, keeping the flame low to prevent curdling.
Add grated coconut (reserving a little for garnish), remaining green chillies, and a pinch of sugar. Cook patiently until the masala thickens and starts releasing oil.
Place the fried fish pieces into the gravy. Coat them gently with the masala.
Add about 1/2 cup warm water, cover, and cook for 5–6 minutes on low heat.
The gravy should turn thick, creamy, and cling softly to the fish. Finish with a drizzle of raw mustard oil on top.
