The Sandwich Rollmops is built around an acidic fish that is already a finished thing before it reaches the bread. A rollmops is a herring fillet pickled in vinegar and rolled around a core of onion and gherkin, skewered to hold its coil, sharp and soft and bracingly sour. The build is a length of bread, often a sturdy split loaf, the rollmops unrolled or sliced into rings along the crumb with its onion and gherkin distributed through, and a thin spread of butter underneath, with little else so the pickled herring is what you taste. This belongs to the Nord, where the cured-herring tradition is strongest.
The logic follows from the acid. A rollmops is intensely vinegared and salty, so the sandwich is mostly about giving that sourness something soft and fatty to land on rather than adding more flavor to it. Butter is the carrier here and earns its place: it coats the crumb, blunts the vinegar's edge, and bridges the sharp fish to the bread the way it would for a salt-cured charcuterie, where a dry-on-sour build would read harsh. The onion and gherkin already coiled inside the fillet do the work a condiment would otherwise do, so the sandwich needs little adding, perhaps a few rings of raw onion if the rollmops ran mild. The bread has to have enough body to absorb the brine without going slack, which is why a firm crust works better than anything soft. There is no warm component and no waiting beyond unrolling and laying the fish; this is assembled fast and eaten plainly, the curing already done by whoever pickled the herring.
Variations move along the cure and the carrier. A sharper, longer-pickled rollmops reads more bracing and wants more butter under it; a milder one is rounder and needs less; a smear of crème fraîche in place of butter softens the whole thing toward the Northern open-face style. Each holds the rolled, vinegared herring with its onion-and-gherkin core as the fixed point and adjusts only the sharpness or what cushions it. The Sandwich Rollmops belongs with the fish builds the catalog groups under Baguette Poisson, the French answer to the Scandinavian open-face fish toast. Its specific contribution is acid as the defining axis: a pickled, self-condimented fish where the sandwich's job is to soften the sour rather than season the fish.