The Sandwich Moutarde de Dijon is a condiment-forward sandwich: the thing it is built around is the mustard, and everything else is chosen to give it room. Moutarde de Dijon is the sharp, smooth, pale-yellow mustard of Dijon, made from brown seed and verjuice or wine, with a heat that hits high in the nose and clears fast rather than lingering. Most sandwiches use it as a stripe to cut richness. This one inverts that, treating the mustard as the lead and the rest of the build as the carrier, the way another sandwich treats a cured meat or a cheese as the reason it exists. The bread is a split baguette, the fillings are the reliable companions, ham, a firm cheese, a cold roast, and the defining decision is that the mustard is laid on with intent rather than dabbed, present in every bite from the first.
The craft is in handling an ingredient assertive enough to wreck the sandwich if it is misjudged. Dijon's heat is volatile and clean, which means a thin even film reads as a bright running edge under the filling, while a heavy careless smear simply scorches the palate and erases whatever it was meant to lift. The fat in the build is what holds it in check: butter under the mustard, or the fat of the ham and the cheese around it, blunts the burn into a sharp warmth and keeps it from turning shrill. The bread matters too, because a strong crust and a real crumb stand up to the mustard's bite where a soft loaf would simply taste of mustard and nothing else. The version that works is a question of proportion, enough mustard that it leads, enough fat that it does not dominate, and it is best eaten soon after assembly before the mustard has fully soaked the crumb and flattened into the bread.
Variations move with whatever the mustard is asked to lift. A moutarde à l'ancienne, the whole-grain version, swaps the smooth sharp film for a coarser, milder, seedier bite; the carrier shifts between ham, cold roast beef, and a firm cheese depending on the counter; a leaf of something bitter sometimes joins to extend the edge. The Sandwich Moutarde de Dijon belongs with the place-named builds the catalog groups under Regional Specialty Sandwiches, where its contribution is to put the condiment in the lead and let the rest of the sandwich answer to it.