Few things in the French pantry are as concentrated as a cured anchovy, and the sandwich built on it stands or falls on how that intensity is handled. The defining element is the salt-cured anchovy fillet, deep brown-red, oil-slicked, savoury to the point of being almost a seasoning rather than a fish. The Mediterranean version usually lays whole fillets, sometimes a marinated white anchovy for a sharper, vinegar-bright note instead of the cured one. The bread is a crusted loaf, split, and the move that makes the sandwich work is the counterweight: a thick film of butter, or just a hard turn of black pepper, set against the cure.
The whole craft is salt management. A cured anchovy carries enough salt and umami to flood a sandwich on its own, so the build is not about adding flavour but about giving the palate somewhere to land. Sweet butter is the classic answer, smoothing the saline edge and carrying it across the crumb the way it carries a slice of ham; the alternative is to lean the other way and let coarse pepper meet the fish head-on with nothing softening it. Either way the discipline is restraint. Pile on a strong cheese or a sharp pickle and you are stacking intensity on intensity until nothing reads. The bread needs a real crust because the filling brings oil and force but no structure, and the sandwich is best eaten within a few minutes of assembly, before the oil works fully into the crumb and the bite goes uniformly soft.
Variations stay close to the cured-fish idea. The same loaf takes the fillets mashed with a little oil into a rough paste smeared thin, the anchoïade register, which spreads the salt more evenly than whole fillets do; or a marinated white anchovy for a brighter, more acidic read; or a single ripe tomato slice laid under the fish so its sweetness and water push back against the cure. Each is a small adjustment to the same intense centre. The Sandwich aux Anchois belongs with the tinned and cured fish builds the catalog groups under Baguette Poisson. Its specific contribution to that shelf is an ingredient strong enough to behave like a condiment, so the sandwich's whole job is to frame the salt and then get out of its way.