Marinate fish pieces with a pinch of salt and a pinch of turmeric.

In a bowl mix mustard powder, turmeric powder, salt, redchilli powder, yoghurt, tomato puree, water and green chillies gently with a spoon

Take oil in a pan and shallow fry the pieces on both the sides till it turns golden.(if you're calorie conscious , you can skip this step)

Take the fish out of the oil and In the same hot oil, add nigella seeds,and once it starts popping , add the mixture.

Let it come to a boil. Then add the fish pieces.Simmer, Cover and cook for 5 mins

Open the lid and serve hot with rice.