Marinate the fish with ½ tbsp of salt and ½ tsp of turmeric powder. Leave them aside for 10 minutes.
Add 4 tablespoons of mustard oil in the kadai (wok) and heat it on a medium flame.
When the oil is hot and smoking, slide the marinated fish pieces carefully.
After one side is done, flip the pieces and fry the other side properly.
Take them out on a plate when done.
Add the nigella seeds.
After the nigella seeds start splattering, add the masala paste and give it a good stir.
Clean the bowl and add the spice-loaded water to the pan.
Add ½ tbsp of salt and 1 tsp of turmeric powder. Stir well.
Continue stirring for 2 minutes on a low flame. (Pro Tip: Fry the spices until the oil separates. This develops a deeper flavor and prevents the curry from tasting raw.)
When the paste releases oil, add the green chilies.
Add 2 cups of water.
After the water comes to a boil, add the fried fish pieces one by one. (Pro Tip: Once the fish goes in, gently swirl the pan instead of stirring vigorously to keep the pieces intact.)
Cover the pan with a lid.
After 5-7 minutes, turn off the heat. The fish curry is ready.