Roast beef and mustard is the same cold-roast-beef sandwich tuned to a different heat. The constant is cold rare roast beef, sliced thin, on good bread; what defines this version is English mustard, and English mustard behaves nothing like the horseradish it is so often set against. Where horseradish gives a volatile pungency that clears the nose and fades fast, English mustard delivers a hot, bright, lingering burn that sits on the tongue and builds. The defining fact is that contrast of heats. Beef is rich and faintly sweet, and English mustard cuts it with a sharpness that stays through the bite rather than flashing and going, which makes this a slower, more sustained sandwich than its horseradish cousin.
The craft is the dose and the cut, and the dose is unforgiving because English mustard is far stronger by volume than it looks. It is spread thin, in a measured film rather than a layer, because a heavy hand turns the sandwich into pure heat and buries the beef it is meant to lift. The roast is sliced thin and against the grain so a cold cut stays tender rather than going to rope, and a marbled sirloin or rib holds its succulence cold where a lean topside dries; with mustard the cut matters more than usual, because unlike a creamed horseradish the mustard adds no fat or moisture of its own to carry a lean slice. Butter on the bread does that lubricating work and also tempers the mustard slightly, bridging its burn to the wheat. A sturdy bloomer or a plain white holds the filling without collapsing under it.
The variations are an argument about which sharp note carries the beef, and mustard is only one answer. Horseradish, creamed or fresh-grated, gives a fading pungency instead of a building burn; raw or pickled onion answers the richness with crunch; peppery watercress brings a green sharpness; crumbled Stilton answers it with salt rather than heat. The mustard itself can be swapped, a grainy wholegrain for texture and a milder bite, though that is a different sandwich rather than a tweak to this one. Each deserves its own article rather than being crowded in here.