Egg and cress is the egg sandwich solved with a shoot. The constant is bound chopped egg, soft and mild and one-note on its own; the variable is mustard cress, the thin seedling with a hot, peppery bite that almost no other sandwich green carries at that intensity for its size. Cress does two jobs the egg cannot do for itself. It puts a sharp pepper note against a flat, fatty filling, and it puts a fine, brittle texture against a soft one, so a sandwich that would otherwise read as a single smooth thing gets both a flavour edge and a textural edge from a garnish-sized handful. That is why the cress is named on the sandwich and not treated as a leaf you could swap for any other.
The craft is the bind, the moisture, and the freshness of the shoot. The egg is chopped and held with just enough mayonnaise to cohere without going to a wet paste, seasoned with salt and white pepper in the bowl, because the whole sandwich rests on that ratio being right. The cress is cut from its punnet at the last sensible moment and used dry, since it is delicate and wilts fast once it is bruised or wet, and a tired, slimy cress is worse than no cress at all. It is laid in an even layer so the pepper and the crunch reach every bite rather than clumping at one end. The bread is soft white, buttered to seal the crumb against the bound egg, pressed and cut clean.
There is a register split worth naming here, because egg and cress lives two lives. One is the made round: a soft sandwich assembled fresh, the cress visibly green, eaten the same hour. The other is the meal-deal triangle, the same filling engineered to survive hours in a chiller cabinet, where the cress is folded into the bind rather than layered so it does not wilt against the bread, and the sandwich is a different object built to the same recipe. Watercress is the leafier, peppery cousin; egg and chive runs an allium instead of a shoot. Each deserves its own article rather than being crowded in here.