Gentleman's Relish is the most concentrated thing on the English store-cupboard shelf, and the sandwich made from it is an exercise in using almost none of it. The relish itself, sold under the name Patum Peperium, is a dark, stiff paste of anchovies pounded with butter, spices, and a guarded blend of herbs into something so salty and so insistent that a knife-tip is a serving. The sandwich is not built around the paste in the way a ham sandwich is built around ham. It is built around the discipline of restraint: a film of relish dragged across buttered bread so faint that it tints the surface brown rather than coating it. Spread it as you would spread jam and the result is inedible, a slab of salt and fish oil with nothing to hold it back. The craft is the smallest possible quantity that still registers, and getting that quantity right is the whole sandwich.
The reason the restraint works is that three components are each doing a separate job and only one of them carries flavour. Butter goes on first and goes on generously, because it is the medium that distributes the relish: spread the paste straight onto dry bread and it sits in a concentrated streak in one place and nothing in the next, but worked thinly over a buttered surface it disperses into an even savoury wash. The bread is soft, plain, and thin, very often with the crusts cut away, because a filling with no texture of its own gains nothing from a chewy crust and is best kept delicate enough that the seasoning is the only event. Pressed and cut small, it holds for hours without weeping or sliding, which is the practical logic of a cupboard paste in the first place: a sandwich that survives a tin and asks nothing of a kitchen. The English tea tray treats it as a savoury counterpoint to cucumber and egg, a single dark note among pale ones, and that is exactly the register it is calibrated for.
Gentleman's Relish sits inside the wider potted-and-paste tradition, the family of pounded fish and meat spreads kept in jars and laid on thin, and within that tradition it is the loudest member and the one used in the smallest amount. The plainly named anchovy paste is its broader cousin, less guarded in its spicing and used the same way; anchovy toast, where the fish is mashed onto a single crisp slice rather than sealed thin between soft ones, is a genuinely different build with its own logic. Those deserve their own articles rather than being crowded in here.