Soak mustard seeds and poppy seeds in water for at least 6 hours. Overnight will work best.
Grind the soaked mustard seeds, soaked poppy seeds, two green chillies and water (as required) to make a thick paste. Set aside.
Heat mustard oil in a pan. Let it heat up and release smoke. Temper with nigella seeds.
Add the mustard and poopy seeds paste. Fry for around five minutes on low to medium flame.
Add fresh ginger paste and mix on low flame till raw aroma of it disappears.
Add 250 ml hot water in the grinder and mix with the leftover paste. Pour it on the pan.
Mix well. Season with salt and roasted cumin powder.
Once it comes to an aggressive boil, add the washed and cleaned pabda fish into the gravy. Be gentle as they might break.
Distribute the fish pieces evenly. Cover and let it cook for around 30 minutes on low flame. Keep checking in between.
Take lid off, drizzle some mustard oil on top and garnish with chopped coriander leaves and slit green chillies.
Serve hot with boiled rice.