The Sandwich Cervelle de Canut is built on a Lyonnais spread whose name promises something it does not contain. Cervelle de canut, the "silk-weaver's brain," has no brain in it at all: it is fresh fromage blanc whipped smooth and worked through with chopped herbs, shallot, garlic, oil, vinegar, salt and pepper, a soft white cheese preparation of Lyon eaten with bread. The sandwich is that spread given a loaf: a split crusted bread, the herbed fromage blanc laid on thick, usually little or nothing else, the cheese carrying the whole thing.
The logic is that the spread is already seasoned and already a filling. The shallot and garlic give it bite, the vinegar gives it acid, the herbs give it lift, so nothing needs adding: it is the savor and the binder at once, gripping the crumb. The constraint is moisture and softness. Fresh fromage blanc is loose and wet, so it has to be the well-drained, thicker kind rather than a pourable one, and even then the bread has to resist it: a firm crust and a crumb with some chew, doing the structure the spread cannot. The window is short, because a wet white cheese will slacken a soft loaf if it sits. There is no heat. This is a sandwich made close to when it is eaten, the spread mixed ahead so the herbs and shallot bloom into it, the assembly done last.
Variations stay within the Lyonnais vocabulary. A version with extra herbs and a sharper hit of shallot pushes the savory edge; one with a few slices of a firm cured meat under the spread adds salt and chew against the soft cheese; the plainest is the herbed fromage blanc on good bread alone, the spread standing as the entire filling. Each keeps the herbed fresh-cheese spread as the fixed point and changes only what sits with it. The Sandwich Cervelle de Canut belongs with the regional-cheese builds the catalog groups under Baguette Fromage. Its specific contribution is a seasoned fresh-cheese spread as the whole sandwich: a build whose quality is decided by how well the fromage blanc was drained and how deep the herbs and shallot went into it.