The defining problem in a Sandwich au Maquereau is an oily fish that is loud at both ends of the flavor, and the sandwich has to be built to carry it without flattening or souring it. Maquereau, mackerel, is a dark, rich, strongly flavored fish from the Atlantic coast, eaten in the sandwich either hot-smoked until firm and dense or cooked fresh and flaked while still moist. Smoked, it is concentrated, salty, and faintly resinous; fresh, it is softer and cleaner but still assertively fishy in the way only an oily fish is. The build is a length of baguette or a split crusted loaf, a spread of butter, the mackerel laid in fillets or broken into coarse flakes, and a sharp acid driven through it: lemon, or thin rings of pickled shallot. What lifts it past a plain fish sandwich is the management of that oil, because mackerel goes either dull or rancid-tasting if the acid and salt are not set against it.
The logic follows from the fat. An oily fish coats the palate and stays there, so it needs a cutting agent built into the sandwich rather than served beside it: the lemon or the pickle is the counterweight that keeps the richness honest through the last bite. The butter is not redundant against an oily fish, it bridges the salt of the smoke to the wheat of the crust the way it does for a sardine. Texture decides the handling: smoked mackerel holds together and can be laid in slabs, while fresh-cooked mackerel is fragile and should be broken into loose flakes and mounded rather than packed, so it stays tender to the bite. The bread needs a firm crust because the filling brings no structure of its own. There is little reason to wait once the acid is on, this is a sandwich made close to service, the work already done by whoever caught and smoked or cooked the fish.
Variations move along the preservation method. Smoked mackerel for a denser, saltier sandwich; fresh-flaked for a lighter, cleaner one. A thin smear of crème fraîche folded under the fish softens the smoke and rounds the oil. A few capers pushed through the flakes sharpen the cut further for anyone who finds lemon alone too mild against the fat. Each is an adjustment of richness against acid, the bread held constant. The Sandwich au Maquereau sits among the cold-water fish builds the catalog groups under Baguette Poisson, the French answer to the Scandinavian open-face fish toast. Its specific contribution is an oily, strongly flavored fish that forces a sharp acid into the build to keep it from going dull.