The Sandwich Entrecôte is built around a cut that has to be cooked rare or the sandwich does not work. The entrecôte is the rib steak, marbled through the muscle with seams of fat that need real heat to render but only seconds of it, seared hard on the outside and left red in the middle, then rested and sliced thin across the grain and shingled onto a split baguette. In the Bordeaux register it comes with a shallot-and-red-wine reduction worked into the build, the sauce that the region's steakhouses keep beside the grill. What makes it its own thing is the doneness: this is a steak sandwich whose entire quality is decided by stopping the meat at rare and slicing it the right way.
The logic is about what marbled beef does after it is cut. Taken past rare, the entrecôte tightens, the fat goes from silky to greasy, and a thin slice turns to a chewy strap that fights the bread; held at rare and sliced across the grain, the same cut stays tender enough to give way at the bite and keeps its juices instead of leaving them on the board. Resting before slicing is not optional, because a steak cut straight off the heat bleeds into the crumb and the sandwich goes soggy in minutes. The shallot-and-wine reduction earns its place by cutting the fat with acid and giving the lean of the meat something to push against. The bread has to have a crust with structure and is best buttered, so the layer of fat seals the crumb against the juices the rested meat still gives up. This is a sandwich eaten soon, while the slices are still warm and the crust still has its bite.
The variations stay close to the cut and the sauce. The same rare-sliced entrecôte takes a pat of maître d'hôtel butter melting into the warm meat instead of the wine reduction, a stripe of mustard for a sharper edge, or a few rounds of shallot left raw for bite. The Sandwich Entrecôte belongs with the beef and roast builds the catalog groups under Baguette Rôti / Bœuf. Its specific contribution is the marbled rib cut as a constraint: a sandwich that lives or dies on the meat staying rare and being sliced across the grain.