Roast beef and Yorkshire pudding is the Sunday dinner rebuilt as a handheld thing, and the structural novelty is the pudding itself. A Yorkshire pudding is a crisp-edged, hollow-centred batter bake that, split or laid flat, behaves like a vessel rather than a slice. Here it goes inside the sandwich: cold or warm roast beef on bread, with a piece of Yorkshire pudding folded in alongside it, so the plate's two defining elements, the roast and the pudding that always sat under its gravy, end up between bread together. The defining fact is that the pudding is doing structural and flavour work at once. It is the savoury, eggy, slightly chewy ballast that catches and holds the horseradish or the thin gravy, and it is the part that makes this read as a roast dinner and not simply a beef sandwich.
The craft is moisture and architecture, because two starches now bracket a cold meat. The beef is sliced thin and against the grain so it stays tender between bread rather than going to rope, and it is laid against the Yorkshire pudding, which absorbs and holds a measured spoon of horseradish or thin gravy where loose sauce on the bread alone would soak straight through. The pudding is best slightly flattened or split so the sandwich still closes, and the bread underneath needs real structure, a sturdy white or a bloomer, because it is now carrying meat and pudding both. Butter to the edges seals the loaf against the sauce. The reward is the contrast the plate had: tender beef, the crisp-then-soft give of the pudding, and a sharp counter pulling the whole thing together in one hand.
The variations are the carvery in order, with the pudding as the constant trick. A spoon of thin gravy turns it into the wettest, most dinner-like reading; horseradish keeps it sharp and dry; English mustard swaps the heat. The same beef without the pudding is the plain roast beef sandwich, and the whole roast dinner rolled inside a giant Yorkshire pudding is the wrap reading rather than this bread one. Each deserves its own article rather than being crowded in here.