Every family that makes Bhapa Ilish will tell you their version is the correct one, and every version is, in fact, slightly different. This one comes from a household near Khulna that has been making it the same way since the grandmother of the current cook first learned it.
A Recipe Passed Down Three Generations
The base is unchanged across all three generations: black and yellow mustard seeds soaked and ground fresh, never from a jar, mixed with green chili, a little coconut, and mustard oil until it forms a loose, pourable paste rather than a thick one.
What's changed is the steaming method — the original used a covered clay pot set inside a larger pot of simmering water; the modern version in the same family now uses a covered steel tiffin box for exactly the same gentle, indirect heat.
The Technique That Makes It Work
The fish is never boiled directly in the paste — it's sealed with the paste inside a covered container and steamed, so the Hilsa cooks in its own fat and the mustard paste's steam rather than in liquid. That's the detail most recipes skip, and it's the one that keeps the fish from turning dry or bitter.
The paste should coat the fish like a blanket, not drown it like a soup.
Belly-cut Hilsa works best here — the higher fat content holds up beautifully to steaming and takes on the mustard flavor without losing its own.