Here the curing carries the flavor, and the sandwich is built around a fish that was finished long before it reached the bread. In the Nord, herring arrives either smoked or pickled, and the two give very different sandwiches. Smoked hareng saur is firm, oily, and deeply savory, the smoke carried right through the flesh. Pickled herring, cured in vinegar and onion, is soft, sharp, and bracing. Either way the fish is not cooked for the sandwich; the work was done in the curing, and the build is an assembly. What lifts it above a generic fish sandwich is that the preserved herring brings its own complete flavor, smoke or acid, and the sandwich's job is to frame an ingredient that is already finished.
The build follows from how much the fish brings. A firm bread, often a dark rye or a sturdy crusted loaf in the northern manner, holds an oily or vinegar-wet filling without collapsing. Smoked herring wants butter beneath it to carry the salt and a few rings of raw onion for bite. Pickled herring brings its own acid and onion, so it wants almost nothing added: bread, butter, the fish, maybe a boiled potato pressed in alongside in the regional way that turns the sandwich into something closer to a plate. Thin slicing off the fillet matters because both the smoke and the cure are concentrated. The sandwich is cold, plain, and unhurried, the kind of thing eaten at a market stall rather than refined at a counter.
Variations stay among the oily preserved fish. The same bread takes smoked mackerel, firmer and a touch sweeter than herring, or oil-packed sardine with the oil shaken back onto the buttered crumb. Rollmops, the rolled pickled herring, gives a sharper, more vinegar-forward version of the same idea. A stripe of crème fraîche sometimes tempers the smoked version where the acid would otherwise dominate. The Sandwich au Hareng belongs with the preserved fish builds the catalog groups under Baguette Poisson. Its specific contribution to that shelf is curing as the flavor: a fish you taste the smoke or the vinegar in before anything the sandwich adds.