The gravlax sandwich is built on a salmon that has been cured rather than smoked, and the distinction is the whole sandwich. Gravlax is raw salmon buried in salt, sugar, and a heavy quantity of dill, pressed under weight until the cure draws out moisture and firms the flesh into something silky, sweet, and herbal, with none of the campfire note that smoking would add. The result tastes clean and faintly aniseed-bright where smoked salmon tastes deep and savoury, and that difference dictates everything around it. The cure is doing the cooking and the seasoning at once, so the sandwich's job is not to flavour the fish but to frame a fish that is already finished, sweet and dilled and dense, against bread that will not overwhelm it.
The craft is slicing and the dressing. Gravlax is cut on the bias into broad, thin sheets so it folds and drapes across the bread rather than sitting as a slab, which keeps the texture silky instead of dense in the mouth. The classic counter is a dill and mustard sauce, a sweet-sharp emulsion that echoes the dill in the cure and answers the cure's sweetness with mustard heat and acidity, spread thinly because it is there to lift the fish, not coat it. The bread is plain and soft, very often a dark rye or a fine brown, chosen because a fish this delicate is lost under an assertive crumb and because rye's faint sourness flatters a sweet cure the way it flatters pickled herring. Butter under the sauce seals the bread and stops a moist filling soaking it, and the whole thing is kept thin and restrained because gravlax punished by too much sauce or too much bread stops tasting of itself.
The gravlax sandwich belongs to the British coastal-fish shelf, the seaside family where the discipline is to do as little as possible to a good catch, and within it it is the cured rather than the smoked member. Its near relatives are set by what dresses the fish: gravlax with dill and mustard sauce reads sweet-sharp and classic, gravlax with crème fraîche and cucumber reads cool and clean, gravlax on rye with capers pushes it toward the briny end. Smoked salmon is a separate build with a separate flavour. Those deserve their own articles rather than being crowded in here.