Roast beef and horseradish is the roast-dinner sandwich whose entire character is decided by one fierce condiment. The constant underneath is cold rare roast beef, sliced thin, on good bread; what makes this version itself is the horseradish, and in its most common form that is creamed horseradish, the grated root folded into cream or crème fraîche so the burn is rounded into something spreadable. The defining fact is the heat. Beef is rich, mineral, and faintly sweet, and on its own between bread it reads heavy and one-note. Horseradish answers it with a sharp volatile pungency that clears the palate between bites, and the sandwich is built around that single jolt rather than around the meat alone.
The craft is the cut of beef and the handling of a sauce that fades. The roast wants to be sliced thin and against the grain, because cold beef cut thick or with the grain goes to rope between bread, and the thin slice is what keeps a cold cut tender. A well-marbled cut such as sirloin or rib carries enough internal fat to stay succulent cold, where a lean topside dries and needs the sauce to carry it; the choice of cut quietly decides how much work the horseradish has to do. Creamed horseradish has a structural advantage worth knowing: the cream coats the bread and binds the slices, supplying the lubrication a cold roast lost on the carving board, while the grated root inside keeps the heat. It is spread in a measured layer, not flooded, because horseradish dulls fast under refrigeration and an over-soaked slice loses both its bite and its structure. A bloomer or a sturdy white holds a heavy filling without collapsing.
The variations are mostly an argument about the horseradish itself, and the sharpest split is creamed against fresh. The fresh-grated, raw-root build is assertive and short-lived in a way the creamed version is not, a separate sandwich made at home from the root rather than spread from a jar, and it deserves its own treatment rather than being folded in here. Beyond that, English mustard swaps one heat for another, raw onion adds a pungent crunch, and peppery watercress brings a green sharpness alongside the cream. Each deserves its own article rather than being crowded in here.