Tuna and cucumber takes the bound tuna baseline and answers it with cool water-crisp. The tinned tuna and its measured bind of mayonnaise are the constant; the variable is the cucumber, and what it brings is not flavour so much as temperature and texture. Cucumber is mild, watery, and cold, and against a rich, salty, slightly heavy tuna mix it reads as a clean break, a snap and a coolness that lightens every bite. This is the gentlest member of the family, the one that does not add a sharp or sweet note but a refreshing one, which is why it sits as comfortably on the afternoon-tea tray as it does in a lunchbox.
The craft is the same bind problem as the baseline plus a second source of water to control, and the second one is the catch. Cucumber leaks heavily once cut, so it is sliced thin, often salted and drained or patted dry, and laid in a single layer rather than piled, because cucumber that weeps into a tuna already carrying its own moisture is what turns this sandwich to a wet, sliding mess. The tuna is bound slightly firmer than usual to give the cucumber's water somewhere to go without slackening the whole filling. The bread stays soft and plain, buttered to the edges as a waterproof layer between a doubly wet filling and the crumb, and the assembly is kept thin so each bite gets both the tuna and the snap rather than one then the other.
The variations are the rest of the tuna family, each leading on a different counter to the same fixed bind: onion for raw bite, sweetcorn for a sweet pop, celery and pepper for the branded crunch, plain mayo for the baseline itself. Cucumber is the one that chooses coolness over sharpness, and the crustless tea-tray version of it is the formal reading. Each deserves its own article rather than being crowded in here.