The mackerel sandwich is built around a strong, oily fish, and the whole construction is a search for the right thing to cut against it. Mackerel is one of the richest fish on the British coast, dense and pronounced even when fresh and more so when smoked or peppered, and on bread it dominates anything timid put near it. Smoked mackerel comes already firm, dark, and intense; the peppered fillet adds a coating of cracked black pepper that pushes the savour further still. The defining decision is therefore the counter, not the fish: a mackerel sandwich is good in proportion to the sharpness set against the oil, and flat in proportion to its absence.
The craft is balance and moisture. The fish is flaked off its skin and either left in pieces or lightly bound, and the counterweight is the part that takes the thought: lemon squeezed over the flakes, a horseradish or mustard smear, a layer of crème fraîche, or pickled cucumber and red onion, each chosen to be acidic or hot enough to lift a heavy, fatty filling rather than disappear under it. Bread with real structure, a brown bloomer or a sturdy granary, is used because oily mackerel needs a crumb that holds up to it and does not go greasy and soft, and the spread is kept measured so it seasons without soaking. A leaf of watercress or a few capers adds the bite an all-soft filling otherwise lacks. The fish carries the flavour on its own, so the rest of the build exists to sharpen it, not to compete.
The variations follow the same oily-fish-against-acid logic. Smoked mackerel pâté blends the fish with cream cheese and lemon into a spread; the peppered fillet leans on its own pepper crust and asks only for bread and a squeeze of citrus; the wider coastal shelf swaps in kipper, sardine, or pilchard against the same sharp counter. Each deserves its own article rather than being crowded in here.