The Sandwich Dijonnais is built around mustard treated as a primary ingredient rather than a faint condiment streak. Dijon mustard, the smooth, pale, sharp paste made with brown seeds and verjuice or white wine, is spread on the crumb thickly enough that it registers as a flavor in its own right, and the rest of the sandwich is assembled to give it something to work against. The frame is a split baguette, a few slices of jambon or a cold roast, sometimes a wedge of a regional cheese, and the Dijon laid on with a heavier hand than a Paris counter would dare. What makes it its own thing is the register: this is a sandwich tuned to the heat and clean bite of the mustard, not one where the mustard hides behind the meat.
The logic is a balancing act. Dijon is volatile and assertive, all top note and clearing burn, so it needs fat and salt opposite it or it overruns everything: butter under the meat, the natural richness of a cold roast, the salt of a cured ham, a milder cheese to round the edge. Applied with restraint the mustard would be a background hum, which is the Paris approach; applied with a Dijon hand it becomes the structural axis the whole sandwich turns on, and the cook's job is to load enough richness around it that the burn lifts the filling instead of flattening it. The bread needs a real crust and a tight crumb, because thinned mustard will soak a soft loaf and the sandwich is best eaten reasonably soon after the mustard goes on, before the edge of the paste has had time to dull.
The variations stay within the mustard register rather than wandering off it. A coarse, whole-grain moutarde à l'ancienne trades the smooth burn for a seedy, gentler texture; a mustard let down with a little crème to take the edge off suits a richer roast; the filling swings between ham, cold beef, and a regional cheese while the Dijon stays the constant. The Sandwich Dijonnais belongs with the place-named sandwiches the catalog groups under Regional Specialty Sandwiches. Its specific contribution is mustard promoted from condiment to lead: a sandwich organized around the sharp clean heat of Dijon and the fat it needs to sit against.