Chicken and watercress is the chicken-mayonnaise tea sandwich whose single aromatic is a peppery green, and unlike its siblings that note comes from a leaf rather than from something stirred into the dressing. Sliced or chopped chicken is bound in mayonnaise as in the rest of this family, and watercress is laid through it as the defining counter: a fresh, sharply peppery cress with a faint mustard bite that cuts the richness of the bind and lifts mild poached meat out of blandness. The pepper is the whole point of the sandwich. Replace the watercress with lettuce and you have a softer, duller thing that no longer earns its own name.
The craft is keeping the cress alive and the structure dry. Watercress wilts fast and bruises easily, so it goes in fresh, dried thoroughly after washing, and ideally close to serving, because a tired, blackened cress loses its pepper and weeps water into the bread. The leaf also adds bulk and a little moisture of its own, so the mayonnaise is kept on the firmer side and used sparingly, just enough to bind the chicken without the two wet elements together turning the filling slack. The chicken is poached gently and kept tender so the peppery green has good meat to lift rather than dry strands to mask. The bread is soft, plain, and buttered to the edges, both to seal the crumb against the cress's moisture and to leave the field clear for a flavour that depends on freshness. Cut crustless and small, it should taste green and clean.
The variations are its own family, each defined by which single aromatic is set against the chicken. Chicken and lemon works bright acid through the bind; chicken and tarragon carries an aniseed herb the same way; the plain watercress sandwich drops the chicken entirely and lets the cress stand alone. Each deserves its own article rather than being crowded in here.