The watercress sandwich is defined by a single peppery leaf and almost nothing else. A bed of fresh watercress on crustless white bread with salted butter, and no second vegetable, no herb, no sauce: it is one of the canonical afternoon-tea fingers, and its whole character is one assertive flavour set against deliberate plainness. Where the cucumber finger leans on coolness and faintness, this one leans the other way, on the brisk, mustardy, mineral pepper of the cress itself. That heat is the point of the sandwich and the reason it needs no help. The build is an argument that a single sharp leaf, handled with restraint, is a complete filling.
The craft is moisture control and the discipline to add nothing. Watercress is delicate and holds water in its leaves and stems, so it is washed, then dried thoroughly, and the coarser stalks picked out, because a wet leaf weeps into the crumb and a thick stem drags out of the side instead of yielding to the bite. Salted butter is structural rather than a topping: spread firmly to the very edges it waterproofs the bread against the leaf and its salt is the only seasoning the sandwich gets, the single counter that lifts the pepper of the cress without competing with it. The bread is soft white sliced thin, the crusts cut away after assembly so nothing with chew resists a filling chosen for its tenderness, and the sandwich is cut into fingers or triangles small enough for two bites without a plate. It is made close to when it is served, because watercress wilts quickly and a leaf that has stood goes flat and loses the very pepper it was chosen for.
The variations are the single additions that each earn a different name. Egg bound soft and mild to carry the cress is the most common, the pepper cutting the richness; cream cheese for a tangy body that fixes the slip; thin chicken or smoked salmon to turn a delicate thing into something heavier. Each is its own pairing rather than a tweak to this one, and each deserves its own article rather than being crowded in here.