Smoked mackerel and horseradish is a pairing before it is a sandwich, and the pairing is the point. Hot-smoked mackerel is one of the richest things the British coast puts on bread: the fillet comes off the smoke already cooked, dark, dense, and heavy with oil, intense in a way that flattens anything timid laid against it. Horseradish is the answer the fish demands. Grated sharp into cream or worked into a smear, it brings a clean burning heat that cuts straight through the oil and lifts the smoke instead of competing with it. The fish is the body and the horseradish is the release valve, and a version with one and not the other is either cloying or thin.
The craft is balance and moisture against a filling that brings its own fat. The mackerel is flaked off its skin in pieces rather than mashed, so the texture stays as flakes and not a slick, and the horseradish is folded through or spread under it in a measured amount, hot enough to register against a heavy fish but not so much that it scorches past the smoke. Bread with real structure carries this best, a brown bloomer or a granary with enough crumb to hold an oily filling without going greasy and soft. A little lemon sharpens what the horseradish starts, and a leaf of watercress or a few rings of pickled cucumber add the bite an all-soft, all-rich filling otherwise lacks. The spread stays restrained throughout, because the fish is already the loudest thing in the sandwich and the build exists to sharpen it, not to bury it.
The variations follow the same oily-fish-against-sharp logic and each earns its own name by changing the counter or the form. Smoked mackerel and crème fraîche softens the heat into something rounder and cooler. The peppered fillet leans on its own cracked-pepper crust and asks only for bread and citrus. The pâté blends the same fish to a whipped paste, a different texture and a different sandwich entirely. Each deserves its own article rather than being crowded in here.