The pâté sandwich is the most French of all the lunchroom sandwiches, in part because it requires the existence of a working charcutier and won't really work without one. A thick slice of pâté de campagne — coarse, peppery, often laced with brandy and bay leaf — is laid on buttered baguette with a few cornichons and a leaf or two of frisée, and the sandwich is done. It does not require a recipe so much as a charcuterie within walking distance.
The pâté itself sits on a spectrum. The campagne is forthright: pork shoulder, liver, herbs, a few peppercorns left whole. The pâté en croûte raises the stakes with a butter pastry around the meat — you slice it instead of spreading. The terrine de canard is denser and more wine-soaked, the rillettes du Mans are pork or duck slow-cooked until they shred and emulsify into their own fat, and the foie gras (mi-cuit, on toasted brioche, with a film of fleur de sel) is what you eat for lunch the day someone else is paying.
The sandwich that uses any of these wants three supports underneath: bread with a real crust, butter or another fat carrier to bridge the meat to the bread, and an acidic counterweight — pickle, mustard, or a small green leaf — to keep the richness honest. Get the proportions right and the sandwich becomes a portable assembly of French food culture; get them wrong and it becomes lunch you regret around 3 p.m.