Roast beef and watercress is the roast-beef sandwich that does its cutting with a leaf. The constant is cold rare roast beef, sliced thin, on good bread; what defines this version is watercress, and in its fullest form it is watercress set against a horseradish cream, the peppery green and the fiery root working the same side together. The defining fact is the nature of watercress's sharpness. It is not a chemical heat like horseradish or a building burn like mustard; it is a clean, mineral, vegetal pepper carried on a crisp leaf, and it lifts the beef while also adding the one thing every other roast-beef build lacks, a fresh living texture against dense cold meat.
The craft is keeping the cress alive and the cream in proportion. Watercress wilts fast and bruises under pressure, so it goes in generously but is pressed lightly, and it is dried well, because wet cress weeps into the bread and a soaked roast-beef sandwich loses its structure. The horseradish cream alongside it is the second sharp note and also the moisture: spread in a measured layer it lubricates a cold cut that lost its juices on the carving board, and the cream rounds the cress's edge while the grated root inside extends its pepper into actual heat. The beef follows the standard rule, sliced thin and against the grain so it stays tender cold, with a marbled cut holding succulence a lean one would not. A sturdy bloomer or plain white holds a heavy filling of meat, cream, and a bulky leaf without collapsing.
The variations divide first on whether the horseradish is there at all. Stripped to beef and cress alone, with no cream and no root, it becomes a leaner, greener sandwich that stands on the leaf by itself, and that build carries its own separate treatment rather than being folded in here. Past that, mustard swaps the cream's heat, raw onion answers the beef with crunch, and Stilton with salt. Each deserves its own article rather than being crowded in here.