Blend together the mustard seeds, cumin seeds, coriander seeds, garlic, ginger, black peppercorns, turmeric and green chilli with little water to a smooth paste.
Marinate the fish pieces with 2 Tbsp of the above blended paste, salt, turmeric and chilli powder for 20 min.
Meanwhile, heat oil in a pan and add cumin seeds and mustard seeds. Let them splutter.
Add the onion and fry until brown.
Add the remaining blended masala and fry on low flame until all the rawness is one and the oil oozes out on the sides.
Add the puréed tomatoes and fry until all the moisture is gone.
Add sufficient water to make gravy and boil for a couple of minutes.
Meanwhile, in a separate pan, add oil and fry the marinated fish pieces until brown on both sides and add to the boiling gravy.
Simmer for 5 min, add the coriander leaves and serve hot.