Salmon and cucumber, in its finest form, is a tea-tray sandwich, and it is defined by trimming rather than filling. This is the version that arrives on a stand cut into crustless fingers, the salmon laid in a thin, even seam and the cucumber sliced to translucence, built so it can be eaten in two bites over a saucer without disturbing the conversation it is served alongside. The defining decision is restraint: not how much salmon goes in but how little, sliced or flaked so fine that the sandwich reads as a pale, cool, delicate thing rather than a fish sandwich with bulk. It sits in the afternoon-tea register, where the point is elegance and proportion, not lunch-sized substance.
The craft is moisture control and the cut. Cucumber is mostly water, so it is shaved wafer-thin, salted, drained, and patted dry before it goes anywhere near the bread, because a single weeping slice slackens a sandwich built to be immaculate. The salmon, whether finely flaked poached fish or a translucent sliver of the smoked sort, is laid in one even layer so no bite is mostly fish and the next mostly bread, and butter spread firm and edge to edge is the structural seal, waterproofing the crumb and supplying the only sharp note the build is given. The bread is thin, soft, and plain, white or brown, and the crusts are cut away after assembly so nothing with chew resists a filling chosen for its lack of resistance. It is made close to serving and pressed lightly, because even a well-drained version holds its brief, clean freshness only a short while and goes limp if it waits on the stand.
The variations are the other delicate fish fingers of the tea canon, each a different pairing rather than a tweak to this one. Smoked salmon with a turn of lemon and pepper is the richer, cured cousin; poached salmon flaked with a little mayonnaise is the softer one; cream cheese in place of butter binds the slip and adds body. The robust mashed-tin version of the same two ingredients is a coarser, lunchbox thing entirely. Each deserves its own article rather than being crowded in here.