Rub some salt and turmeric powder all over the cleaned fish pieces.
Heat mustard oil in a pan.
Add the fish pieces in oil when it's smokey hot.
Fry them on medium flame for 3-4 minutes by changing the sides.
Take them out from pan.
In that same oil saute nigela seeds.
Then add onion paste, cook for 5-7 minutes.
Then add ginger garlic paste. Cook for 2-3 minutes.
Add poppy seed paste (soak the poppy seeds in hot water for 1 hour, then grind it to a fine paste). Cook for 4-5 minutes.
Beat the yogurt with some water, reduce the flame to low and add in the curry. Mix well and cook for 2 minutes on low flame.
Then add salt, sugar, turmeric powder and red chili powder. Mix well and cook for 2 minutes.
Now add the mustard paste (soak black and yellow mustard seeds in water for 1-2 hours, then put them in a grinder along with 1 green chili, some salt and water. Grind them to a fine paste). Or you can also add Kasundi. Mix well and cook for 2 minutes.
Add hot water, mix well and bring the curry to a boil.
Add the fried fish in that boiling curry, cover the pan and cook for 3-4 minutes on medium flame.
Lastly add slitted green chilies and 2 Teaspoons of mustard oil. Mix well and turn off the heat.
Let the curry rest for 15 minutes then serve.