Wash and smear the Koi fishes with turmeric powder and salt and 1/2 tsp oil, set aside for 15 mins

In a wok heat 2-4 Tbsp oil and add the fish pieces, on medium flame fry them lightly, set aside

Soak cumin seeds with 3 Tbsp water for 10 mins, add to blender and make a fine paste, set aside

In the wok add rest of the oil and add nigella seeds, slit green chilies and stir for 1 min, add cumin paste and on low flame stir for 3-4 mins until well combined

Add mustard paste and turmeric powder, red chilli powder, salt and mix well.

Add fishes and mix well, add hot water and let it come to boil. Cook uncovered for 5 mins