Roast beef and watercress, in this reading, is the build stripped to two things. The constant is cold rare roast beef, sliced thin, on good bread; what defines this version is that the watercress is the only condiment, with no horseradish cream, no mustard, no sauce of any kind. That austerity is the whole point and the line that separates it from the fuller build, the one where the cress arrives alongside a horseradish cream and which carries its own separate treatment. Here the leaf has to carry the entire job of cutting a rich meat on its own, and the defining fact is that watercress is strong enough to do it. Its clean mineral pepper is assertive in a way most salad leaves are not, and the sandwich is an argument that beef needs nothing more than good bread and the right leaf.
The craft is quantity and dryness, because with no sauce the cress is both the seasoning and the only moving part. It goes in generously, far more than a garnish, since a thin layer disappears against dense beef and the leaf has no cream backing it up. It is dried thoroughly and pressed lightly, because without a waterproofing layer of horseradish cream the only thing between wet cress and the bread is the butter, and a soaked slice collapses fast. The beef follows the rule every roast-beef sandwich follows, sliced thin and against the grain so a cold cut stays tender, and here the cut matters more than usual: with no creamed sauce to lubricate it, a lean dry topside has nothing to carry it and a marbled sirloin or rib that stays succulent cold is close to required. Butter to the edges does the lubricating and the waterproofing both. A sturdy bloomer or plain white holds the filling without going limp under a bulky leaf.
The variations begin exactly where this one draws its line. Add a horseradish cream and it becomes the fuller, fierier build rather than a richer setting of this one; add mustard for a building heat, raw onion for crunch, or Stilton for salt, and each takes the beef somewhere else. Each deserves its own article rather than being crowded in here.