The Sandwich Truite Fumée is built around a freshwater fish rather than a sea one, and that distinction is the whole point. Smoked trout is leaner and more delicate than smoked salmon, paler in colour, gentler in smoke, with a clean river-fish sweetness that a heavy hand will flatten. The build is a length of good bread, a film of cold butter or a thin spread of fresh soft cheese, the trout laid in unhurried flakes, and a few small sharp notes: lemon, cracked pepper, dill or chive, sometimes a thread of horseradish. The discipline is restraint, because the trout brings less salt and less oil than its sea-fish cousins and is easily lost under anything assertive.
The logic follows from how lean smoked fish behaves against bread. Trout carries little fat of its own, so it needs a carrier the way a dry cure needs butter: the cold butter or the soft cheese bridges the fish to the crumb and replaces the richness the fish does not supply on its own. The flesh flakes rather than slices, which means it should be laid in loose folds rather than packed flat, so each bite stays light instead of compressing into a dense band. Acid is structural here, not decorative, since a squeeze of lemon lifts the smoke and keeps the fat carrier from going flat across the length of the sandwich. The bread wants enough crust or grain to stand up under the soft fish without competing with it, which is why a seeded or country loaf reads better here than a soft white one.
Variations move along the water the fish came from. The smoked salmon traditions, on a baguette or on rye, run oilier and saltier and louder, and the bagel-and-cured-salmon build is a denser, chewier relative; those each get their own treatment in the catalog rather than being unpacked here. Within the trout register the turns are small: horseradish folded into the cheese for heat, pickled shallot or caper for a sharper acidic note, a seeded rye in place of a country crust to push the sandwich toward the Northern European register the smoked-fish tradition shares. The Sandwich Truite Fumée belongs with the smoked and tinned fish on bread the catalog groups under Baguette Poisson, the French answer to the Scandinavian fish toast. Its specific contribution is a lean freshwater smoke that asks the sandwich to add a fat carrier and an acid, and then to stay out of the way.