The Fish Fry Sandwich puts a Bengali fish fry between bread. The fish fry it is built on is a Kolkata institution in its own right: a fillet, often bhetki, coated in spiced crumb and fried to a flat golden cutlet, served with mustard. Putting that cutlet in bread makes it portable, and the sandwich lives or dies on getting the fry itself right. The bread is a supporting player, usually soft white loaf bread, present to make the fry handheld rather than to add flavor of its own.
The fry comes first and follows its own discipline. A boneless fillet is marinated, then given a crumb coat and fried flat, not balled, so it reads as a slab of fish rather than a croquette. The Bengali signature is mustard: kasundi, the region's sharp fermented mustard sauce, either marinated into the fish or served against it, and that pungency is the whole character. To build the sandwich, the fry is laid on bread with onion and a streak of kasundi or chutney, the bread sometimes lightly buttered. Good execution is a fry that stays a clean flat slab, crust intact and shattering, the mustard cutting the oil so the bite finishes sharp rather than heavy. Sloppy execution is a fry gone limp from sitting, a crust that turns to paste against the bread, or a timid hand with the mustard that leaves the sandwich tasting only of fried crumb.
Where it shifts is mostly in the mustard and the heat. Some versions push the kasundi hard for an aggressive, nose-clearing bite; others temper it with a sweeter chutney or extra onion for balance. The choice of fish moves the texture too, a firmer fillet holding its slab shape better through the bread than a flaky one. The fish chop, a rounder croquette, and the coastal fish cutlet sandwich are siblings on the same fried-fish branch, and each deserves its own article rather than being crowded in here. What pins this one to Kolkata specifically is the mustard: take kasundi out and it stops being a fish fry sandwich and becomes a generic fried-fish roll.