The smoked mackerel pâté sandwich takes the same fish as the flaked-fillet version and changes its physical state, and that change is the whole sandwich. Instead of breaking hot-smoked mackerel into pieces, it is beaten smooth with cream cheese or butter, lemon, and pepper into a soft, spreadable paste. The texture is now uniform where the fillet version is layered, and that is the defining distinction: one is flakes you bite through, the other is a whipped paste you spread. The pâté reads richer and more even, the smoke distributed through every mouthful rather than arriving in pieces, and the build is arranged around that smoothness rather than around protecting a flake.
The craft is the ratio and the bread. A pâté that is all fish is too dense to spread and too strong to eat in quantity, so it is cut with enough cream cheese or soft butter to make it lift on a knife while still tasting clearly of mackerel and smoke. Lemon is worked through it rather than squeezed over, because the acid has to reach the whole paste to keep a rich, oily spread from turning flat. It goes on the bread thin, the way any strong potted spread does, because a thick layer of something this rich slides and cloys. The bread is soft and plain so it carries rather than competes, and butter under the pâté bridges the salt and seals the crumb against the oil, since a smooth filling has no structure of its own to hold a heavy crust at bay.
The variations stay inside the smooth-spread frame and change the cut or the counter. A version bound with crème fraîche rather than cream cheese is looser and cooler, the smoke lighter on it. A little grated horseradish beaten in brings a heat the plain pâté lacks, the same partnership the flaked fillet uses, met in paste form. The flaked smoked mackerel sandwich is the texture untouched, the fish left in pieces rather than whipped, a different sandwich on the same fish. Each deserves its own article rather than being crowded in here.