Cut it thin and across the grain or the whole thing fails, because one cut of beef is being asked to stand on its quality alone. Charolais is the white-coated cattle breed of Burgundy, raised for lean, fine-grained, deeply flavored meat, and the sandwich treats a slice of it as the entire reason to make the thing. The build is a length of baguette, a thin spread of beurre demi-sel or a stripe of mustard, and Charolais beef cooked rare to medium-rare, sliced thin across the grain, and laid in shingles along the bread.
The logic follows from the meat. Because Charolais is lean rather than marbled, it does not flood the bread with fat the way a fattier cut does, so it stays clean in the hand and the bread keeps its structure through the sandwich. That same leanness means the beef carries the flavor on its own savor rather than on richness, which sets the build: a thin smear of mustard or a few cornichons for an acidic break, and little else, since a heavy sauce would mask a meat chosen for its own taste. Slicing thin and across the grain matters here. Cut thick, lean beef goes firm and chewy between bread; cut thin and shingled, it stays tender and folds against the crumb. Cooked past medium it dries out, since there is little fat to keep it moist, so it is kept pink on purpose.
The bread needs a real crust because the filling brings no structure of its own, and the beef is best barely warm or at room temperature, where it stays tender and its flavor reads fullest. A leaf of frisée or a few shavings of horseradish sharpen it without competing; the discipline is to let the cut speak.
Variations move along the Burgundian and neighboring beef rack: a slice of rare bavette for a looser, more open grain, a thin-cut onglet for a more mineral reading, or shredded wine-braised beef when there is leftover boeuf bourguignon to spoon onto the bread. Each is the same idea met in a different cut or cooking. It belongs with the roast and beef sandwiches the catalog groups under Baguette Rôti / Bœuf, and its specific contribution is a lean Burgundian beef that keeps the sandwich clean and asks only to be sliced thin and kept rare.