Doi Ilish

Ingredients
3lb of Hilsa/Ilish fish (cut in Bengali style pieces not steak pieces). shad fish can be used a substitute
1 ½ cup Yogurt
4 tsp Mustard Paste (a wet paste of mustard & green chilies)
1 tsp Ginger Paste
4 or more Green Chillies
¾ tsp panchphoron (for tempering)
½ tsp Turmeric powder
Mustard Oil - preferred for a fish like Hilsa but you can use other oil too
Salt
Preparation:
Wash the fish well, pat dry and rub the pieces with about ½ tsp of turmeric powder a little salt and keep aside.
Heat oil in a Kadhai/Deep Frying Pan/Wok. When the oil is piping hot reduce the heat and slowly slide the fish pieces into the oil. There is going to be a lot of sputtering so be careful. The fish pieces should not be on top of each other, they should remain side by side in the hot oil. So do not add all fish at the same time. Once you have slid the fishes, raise the heat
Once the fish is fried to a light golden yellow (with hilsa very little frying is needed, be careful that the fish does not get fried too much) take it out and drain on a paper towel
In a bowl mix the yogurt with the mustard paste.
Discard the fishy oil if you wish and heat some fresh oil. With hilsa however the mustard oil in which the fish is fried holds a special value for most Bengalis and we dare not throw it out.
Heat oil now, for tempering. Temper with panchphoron and green chilies and wait for the spices to pop. Add the fresh ginger paste.
Lower the heat and the yogurt-mustard sauce
Add salt and let it simmer for a couple of minutes and add water(about 1 cup)
Add the fish pieces and let the gravy simmer and reduce to desired consistency
Cook till you get gravy of the right thickness.