Heat some mustard oil in a pan. Clean the fish pieces and smear with salt and turmeric powder

Once the oil is hot fry the pieces and keep it aside

Parallely you can soak the mustard seeds, cumin seeds, garlic and red chillies. After 10 mins grind this to a paste

In the pan add some more oil and fry the onions and tomatoes chopped into fine pieces. Once it is golden brown add the masala paste prepared and saute well

Once the masala starts leaving oil add some water and salt as per taste and bring to boil

Add the fried fish pieces to the gravy and remove once the gravy is of the consistency you like

Your fish mustard gravy is ready to be relished with plain hot rice