For this preparation you need fresh water fish, in bihar we mostly use fresh water fish (there are no sea side near by but plenty of ponds and ofcorse the mighty river Ganges) so it's either Rohu / Katla -from Carp family .
So for this fish curry in mustard gravy you need.
Fish -1 kg , cut horizontally
Mustard oil - 1/2 cup
Yellow mustard seeds - 2 table spoon
Garlic - 10 pods
Red chillies -4 /5
Tomato- 1 large, chopped finely
Grind mustard seeds , garlic and chillies into fine paste.
Heat mustard oil to smoking point, turn down to medium and fry each piece 2-3 minutes each side , keep adjusting the oil temperature to ensure all the pieces are golden and not over / under cooked .
Keep the fried fish aside on a kitchen towel to drain all excess oil.
Now add all the remaining masala to mustard oil and cook it on very low heat for 10-15 mins , you don't have to stir continuously but ensure that it doesn't stick
When masala is almost done add chopped up tomato pieces and cook for further 5 mins , till oil starts coming out from masala.
Add water, increase temperature and bring to boil.
Turn heat back to medium and leave it cooking for 5 mins.
Now gently add the fried fish pieces and cook for further 3-4 minutes .
Turn off the heat, your fish in mustard curry is ready. Serve it with plain boiled rice.
Also, a good idea to prepare this dish some time in advance (30 mins to an hour) so it gets time to rest and fish can absorb all the masalas.
Mum tells me when they would cook this in winter, the leftovers were covered in a wok and left overnight to get the 'winter chill', it would taste all the more better !