The Sandwich Hareng-Pommes de Terre is a cold plate from the Nord folded into bread. The pairing is fixed and regional: hareng fumé, smoked herring, oil-soft and assertively cured, laid over cooked potato, the two eaten together as a salade de hareng across the north of France. The sandwich is that plate moved onto a crusted loaf, the herring filleted and laid along the bread, the potato sliced into rounds beneath or beside it, sometimes a little oil from the fish carried across. What defines it is the contrast the regional plate is built on: an intense oily smoked fish set against a bland starchy potato that exists precisely to absorb and soften it.
The logic follows from that pairing. Smoked herring on its own is too salty and too oily to carry a sandwich, and the potato is the answer, a mild, dense buffer that soaks the fish oil, carries the cure across the bread, and keeps each bite from being all smoke and salt. The potato is structural as well as a flavor counter: it bulks the soft filling and gives the sandwich body the flaking fish does not. Because both elements are soft, the bread has to have a real crust to stand against them, and the sandwich is best soon after assembly, before the oil works fully through the crumb. A few rings of raw onion or a thread of vinegar adds the sharp note the regional salad uses to cut the richness. It is a cold sandwich by design, oily, savory, and northern, eaten the way the salad is eaten, plainly and without heat.
Variations track the northern fish plate. Hareng saur, the harder dried-and-smoked herring, pushes it saltier and chewier; a soused or marinated herring trades smoke for vinegar sharpness and reads brighter; a smear of crème or a few rings of onion folded in soften and lift it. Each is a recognizable adjustment of the same smoked-fish-and-potato idea. It belongs with the fish sandwiches the catalog groups under Baguette Poisson, and its specific contribution is a fixed regional pairing, the smoked herring carried and tamed by the potato beneath it.