Seen from the bread rather than the jar, the bloater paste sandwich is an exercise in how little you are allowed to do. The paste is fixed: a pounded, cold-smoked herring spread, oily and dark and aggressively marine, an East Anglian larder staple. Everything the maker decides happens around it, and almost every decision is a decision to subtract. No second filling, because the paste already overwhelms one. No sauce, because it is already its own seasoning. No assertive bread, because the fish needs a foil, not a competitor. The sandwich is defined by the restraint the paste demands of whoever assembles it.
The craft is the spread thickness and the butter, and both are dictated by the herring. The paste goes on as a thin, even film rather than a layer, because a generous spread tips a sharp, smoky filling into something that coats the mouth and finishes for too long. Butter to the edges of soft white or brown bread does two jobs at once: it bridges the paste's salt to the wheat and it cushions the oil so the sandwich reads as savoury rather than slick. There is no heat to manage and no structure to brace; this is a cold, pressed, cut sandwich built for a lunch tin, and its quality is decided entirely by getting one ratio right. A trace too much paste and it is all fish; a trace too little and it is buttered bread with a rumour of herring.
Other potted sandwiches sit alongside it on the same logic at lower volume. The beef and chicken paste sandwiches are mild enough to spread freely; the crab and salmon paste sandwiches bring a sweeter fish to the same potted idea; gentleman's relish pushes the concentrated-anchovy version further still. Each deserves its own article rather than being crowded in here.