This is a Bengali staple/favourite, and is cooked with a large variety of sweet-water/river fish that is common in Bengal (Koi carp, carp, Hilsa, Pabda, Bata and so on). Living in a coastal seaside city like Chennai means I don't get these fishes in local markets, and am instead faced with more salt-water varieties such as tuna, mackerel, and perch.
However, I tried the traditional Bengali style fish in mustard sauce on the Emperor/Seer Fish of Chennai - known as the Indo-Pacific mackerel in the West and Vanjaaram in local markets - and found that it was equally successful. Here is the recipe.
Seer/Emperor Fish - 6 slices
Mustard Seeds (quarter cup)
Green Chillies - 8 (cut into slits)
Onions - 2-3 medium, chopped finely
Coriander Leaves - a sprig
Salt to taste
Wash and clean the slices of emperor fish and marinate with turmeric and salt. Shallow fry the pieces in oil, turning both sides until well done. Keep aside.
In the food processor, mix mustard seeds, 4 green chillies, a little water, and grind until it forms a thick paste. Add vinegar to take away some of the pungence of the ground mustard.
Heat oil in a wok and fry onions and chillies until the onions are transclucent. Add salt and a pinch of turmeric powder and stir fry. Add the mustard paste and stir. Add water slowly and bring to boil.
Add fish pieces and let the gravy thicken.
Garnish with coriander leaves and serve hot.