The roast-beef sandwich in France is a quieter cousin of the British rosbif and the American deli pastrami. The French rosbif is cooked to a pink centre, sliced thin, laid in shingles on buttered baguette, and dressed with maybe a stripe of Dijon and a few cornichons. It is restrained where the British version is generous and the American is theatrical, and the restraint is the point.
The variants take the same technique into other cuts. The Sandwich Magret de Canard uses thin-sliced duck breast, often with a smear of fig confit, and lives most happily in the Périgord and Gascogne. The Sandwich Bavette d'Aloyau — flank steak, cooked rare, sliced across the grain — shows up on the bistro lunch menu in Lyon and Paris with a shallot-and-red-wine sauce already on the bread. The Sandwich Boeuf Bourguignon is a Tuesday-lunch special at brasseries that have leftover boeuf from the weekend; the wine-braised beef gets shredded onto crusty bread with a spoonful of its braising liquid, and the sandwich is half soup, half meal.
What all of these share is that the meat carries the sandwich. The bread is a stage; the mustard or sauce is a frame; the butter is a glue. The cook's only job is to keep the meat tender and the bread fresh, and the sandwich largely takes care of the rest.