The poached salmon sandwich is the cold, gently cooked fish, and it is a different sandwich from the smoked one. The salmon is poached in barely simmering water or court-bouillon until it just sets, then cooled and flaked, so it is mild, soft, and clean-tasting rather than salt-cured and dense. That is the defining distinction. Smoked salmon is sliced and assertive; poached salmon is flaked and delicate, and the whole build is arranged around protecting that delicacy rather than dressing it up.
The craft is moisture and the lightest possible binding. The fish is flaked, not mashed, and held with just enough mayonnaise to make it cohere without turning to a wet paste, because the appeal is distinct flakes of soft salmon and over-mixing destroys it. The classic counter is cucumber, salted and drained so it does not weep into the bread, adding a cool water-crisp note against the rich fish; a few drops of lemon and a little black pepper lift it without overwhelming a mild filling. The bread is thin and soft, white or brown, buttered to the edges so the crumb is sealed against a slightly wet filling, because a delicate fish wants a yielding carrier and a heavy crust would bully it. Pressed and cut, often with the crusts off, it sits closer to the afternoon-tea register than the lunchbox one.
The variations stay restrained, in keeping with a filling that does not want much done to it. Cucumber is the standard partner; dill or watercress adds a green note; a thin smear of cream cheese in place of mayonnaise makes it richer and firmer. Its near relatives are the smoked salmon and gravlax sandwiches, the same fish cured rather than poached, and the wider coastal-fish shelf of crab and prawn handled with the same get-out-of-the-way discipline. Those deserve their own article rather than being crowded in here.