The Sandwich Bœuf-Moutarde is the cold roast-beef sandwich whose defining element is named in its own title: the mustard is not a garnish here but half the build. Cold roast beef, cooked to a rosy centre and sliced thin, is laid on a buttered loaf over a deliberate stripe of mustard, usually a sharp Dijon. The beef supplies the body and the mustard supplies the edge, and the sandwich is the deliberate pairing of the two rather than meat with a condiment added as an afterthought.
The logic follows from the contrast. Cold beef sliced thin is tender and deeply savory but, left on its own, slightly flat and one-noted against bread. The mustard is the structural answer: its sharp, hot acidity cuts the meat's richness and lifts the whole sandwich, doing the work that a sauce does in a braise. Butter sits underneath as the bridge, gluing the lean slices to the crumb and keeping a dry cut from chalking against the crust. The constraint is balance and the knife. Too little mustard and the sandwich goes dull; too much and the heat buries the beef it is meant to lift. The beef has to be sliced thin enough to fold, because thick cold slices turn chewy and pull out in a slab. The bread is a stage and a frame, so a fresh loaf with a real crust is enough; the interest is the meat-against-mustard tension, not the loaf.
Variations move the same pairing across mustards and cuts without changing the logic. A wholegrain mustard gives a coarser, milder heat and a popping texture; a stronger cut of beef leans into the savor and asks more of the mustard to balance it; a few cornichons add a second acidic note alongside the first. Each holds the cold beef and the mustard as the fixed pair and changes only their register. The Sandwich Bœuf-Moutarde belongs with the roasted and braised-meat sandwiches the catalog groups under Baguette Rôti / Bœuf. Its specific contribution is a roast-beef sandwich that puts the mustard on equal footing with the meat, built around the tension between the two.