The cucumber sando is the lightest thing in the Japanese sandwich case: thin-sliced cucumber and a film of mayonnaise between two slices of crustless shokupan. It is almost nothing, and that restraint is the appeal. Cool, faintly green, gently salted, it is the sandwich for a hot afternoon or a quiet accompaniment rather than a meal that fills you. The cucumber gives crunch and a clean watery freshness; the mayonnaise adds a thin savory richness and just enough salt to wake it up; the soft milk bread frames both without competing. None of the three is interesting on its own, a slice of cucumber, a smear of mayo, a piece of plain bread, but layered cold and cut clean they add up to something deliberately spare and refreshing.
The craft is all about water, because cucumber is mostly water and water is the enemy of soft bread. A good one uses crisp cucumber sliced thin on a mandoline, salted briefly and patted or pressed dry so it stops weeping before it ever meets the crumb. The bread is fine-grained shokupan with the crusts trimmed for a clean rectangle, often spread edge to edge with a thin layer of mayonnaise that does double duty as flavor and as a moisture seal against the bread. The slices are shingled to cover the whole face evenly, seasoned lightly, then the sandwich is pressed gently and cut soon after building so it eats at its crispest. A good one is cool and snappy with bread that is still soft and dry. A sloppy one is a wet gray slick where unsalted cucumber has bled into the bread, or so thinly filled it is mostly shokupan with a green hint and nothing to crunch.
The variations stay within that quiet register. Some fold a little wasabi or karashi mustard into the mayonnaise for a sharp nasal lift; some scatter sesame or a pinch of salt flakes for texture and depth. A shiso leaf or a thread of umeboshi turns it herbal and tart without weighing it down. Cream cheese in place of mayonnaise makes it richer and closer to a tea sandwich. Push it much further with egg, ham, or a pile of salad and it stops being this featherweight thing and becomes a deli-style sando, which is a different appetite entirely and deserves its own article rather than being crowded in here.