Salmon and cucumber, in its everyday form, is a tin and a cucumber turned into a sandwich, and the tin is what defines it. Most British households did not poach a side of salmon for a weekday lunch; they opened a can of pink salmon, and the sandwich is built around what that gives you. Tinned salmon comes already cooked, soft, slightly salty, and carrying its own oil and a few soft bones, so the work is not cooking the fish but managing it: mashing it down, fork-flaking it with its liquid, and pairing it with cucumber so the result reads as fresh rather than tinned. It is the thrift version of a fish-and-cool-vegetable pairing, and the cucumber is doing more than garnishing, it is the thing that lifts a can into something that tastes made.
The craft is draining and the bind. Tinned salmon brings its own moisture and the cucumber brings far more, so the fish is drained and lightly mashed, mashable bones and all, then bound with just enough mayonnaise or butter to hold it without going to a wet slick, while the cucumber is sliced thin, salted, and patted dry so it cannot weep through. Brown bread is the usual carrier because its closer crumb stands up to a damp filling better than soft white, and it is buttered to the edges so the crumb is sealed against whatever water gets past the draining. A few grinds of pepper and sometimes a squeeze of lemon cut the slight tin note and the oiliness; the cucumber supplies the cool, water-crisp counter against a soft, faintly salty fish. Made well it tastes clean and economical; made carelessly it is grey, wet, and unmistakably from a can.
The variations run along what the fish is and how fine the build gets. Poached salmon, gently cooked rather than tinned, makes a milder, flakier sandwich of its own. Smoked salmon takes it to the cured, assertive end. The crustless tea-tray version of the same pairing is a finer thing again, trimmed and delicate rather than mashed and robust. Each deserves its own article rather than being crowded in here.