Swap the roast beef for wagyu and the sando changes character entirely. The structure is the same as the standard roast beef sando, thin-sliced roast beef folded onto soft shokupan with a horseradish-leaning mayo or light gravy, but the beef is heavily marbled wagyu, and that single substitution turns a measured deli sandwich into a deliberate indulgence. The fat is the whole story here, and it rewrites every other element around it.
The frame is the familiar one and the same craft applies. Shokupan gives a fine, soft, faintly sweet crumb that yields to the meat; the beef is roasted rare and sliced thin and folded, not stacked, so it stays tender across the bite. The wagyu is the differentiator, and it brings its own demands. Marbled beef carries far more intramuscular fat, so it reads as tender and rich even cool, but it also means the cook has to keep it well short of done, because pushing wagyu past medium-rare renders the fat out and wastes the very thing that justifies it. The sauce job shifts too: against this much richness, the horseradish has to lean sharper to cut through, and the quantity of mayo can drop, since the beef supplies its own lushness and does not need a moisture layer the way leaner roast beef does. A good one has wagyu so soft it nearly gives way without chew, sharp horseradish keeping the richness from going heavy, shokupan intact, clean folds visible at the cut. The failures are wagyu overcooked until the marbling is gone and you are left with expensive grey beef, or too timid a horseradish, which lets the fat sit cloying with nothing to lift it.
When it lands, the experience is plush. The bread nearly disappears, the beef is buttery and yielding, the horseradish flares clean and bright against it and fades. It eats richer than its modest size suggests, and a small portion goes a long way, which is the point of building it this way.
This is the indulgent fork of a sando defined by the grade of its beef; the standard roast beef sando is the leaner, more everyday reading of the same idea. That baseline version is distinct enough in eating experience that it deserves its own article rather than being crowded in here.