Egg and watercress is the egg sandwich cut with a leaf rather than a shoot, and the choice of leaf is the sandwich. The constant is bound chopped egg, soft and mild and fatty; the variable is watercress, a peppery green with real body and a mineral, mustardy heat that stands up where a softer salad leaf would simply disappear into the egg. Watercress is doing the same job cress does, putting pepper and freshness against richness, but it does it with structure: whole sprigs with stem and leaf that hold their shape against the bound filling and give the bite something to push through, a physical counter to softness as much as a flavour one.
The craft is the bind and the handling of a leaf that will not wait. The egg is chopped and held with just enough mayonnaise to cohere without slumping, seasoned with salt and white pepper in the bowl, because that ratio is the part of the sandwich with nowhere to hide. The watercress is the variable that fails fastest: it bruises and wilts quickly, so it is washed, dried thoroughly, and the coarsest stems picked out, then laid in as a single even layer close to serving rather than packed in early where it sweats against the egg and goes dark and limp. Dry is the operative word, because wet watercress carries its own water into the bread and undoes the point. The bread is soft and plain, buttered to seal the crumb, pressed gently so the sprigs stay intact rather than crushing flat.
The variations are the rest of the greens-and-egg shelf, each a different counter to the same bound constant. Mustard cress brings the same pepper as a finer, more fragile seedling; chive brings a clean allium line instead of a peppery leaf; tomato trades the leaf for acidity and a water problem. Each deserves its own article rather than being crowded in here.