Ilish, the hilsa, lives in the sea and swims up the rivers of the delta to spawn in the monsoon. The fish caught in fresh water are fattier and more prized, and a certain kind of Bengali will tell you which river yours came from by taste. The Padma is the one people boast about.
The mustard is ground raw with a little salt and green chilli, so it keeps its bite. The fish goes in with it, raw mustard oil is poured over, and it cooks gently. In the bhapa version it steams in a covered tiffin box set inside the rice pot.
It is extremely bony. There is a technique of working the flesh off the fine bones with the fingers and a little rice, and that technique is the reason nobody eats ilish with a fork.
The old ghoti and bangal argument, between families from the west of Bengal and those from the east, has a food front: ghotis back chingri, the prawn, and bangals back ilish. After a Kolkata derby, the winning side is said to buy the corresponding fish.
