The Sandwich Céréales is named for its bread, not its filling, and that is the whole point of it. The base is a multigrain, seeded loaf: a dough carrying a mix of grains and seeds, often seigle, oats, flax, sesame, sunflower, baked into a bread that is denser, nuttier, and more assertive than a plain baguette. Whatever goes inside is secondary to that choice. The sandwich is defined by reaching for a bread with character of its own and then building lightly on top of it.
The logic is that the bread is the loudest ingredient. A seeded multigrain loaf brings its own toast and nuttiness and a chew that a white crumb does not have, so the filling is asked to work with that rather than overpower it. This is why the bread suits the fillings it tends to carry: smoked fish, fresh white cheeses, the kinds of vegetables that need an assertive base under them rather than a neutral one. The constraint is that a dense seeded crumb is heavier and less forgiving than a baguette, so the filling stays on the lighter, fresher side and the proportions stay modest, or the sandwich turns into a brick. There is no required heat. This is a boulangerie sandwich, made on bread baked that morning and best eaten while the crumb still has its bite, not after a day under refrigeration has dulled it.
Variations follow the filling rather than the form, since the form is fixed by the bread. A version with smoked salmon and a fresh cheese leans into the Northern European register the seeded crumb invites; one with a vegetable-forward filling treats the bread as the substance and the filling as the freshness; the plainest is the loaf with butter and a single good ingredient, trusting the grain and seed to carry it. Each keeps the multigrain seeded bread as the fixed point and changes only what rides on it. The Sandwich Céréales belongs with the plant-forward builds the catalog groups under Sandwich Végétarien. Its specific contribution is the bread as the defining decision: a sandwich that starts from the loaf and lets everything else follow.