The Sandwich Brandade is built on an emulsion, not a slice. Brandade is salt cod poached, flaked, and beaten with olive oil and warm milk until it turns into a pale, smooth, almost whipped purée, the cod and the oil bound so completely that the fish reads as texture rather than as a fillet. In the Languedoc, around Nîmes, the version is held close to that base: cod and oil, sometimes a little garlic, no potato to soften it. Spread thick on bread, it makes a sandwich whose filling is a paste with the body of a rillette and the salt of the sea, which is what separates it from a plain tinned-fish sandwich.
The logic follows from the emulsion. Because the cod is already broken down and bound with oil, the filling behaves like a spread: it grips the crumb, it does not slide, and it carries its own fat so the bread needs no butter underneath. The cure does the seasoning, which sets the constraint, since brandade is assertively salty and faintly garlicky on its own and a loud condiment only fights it. The right build is close to bare: a length of crusted bread, a generous layer of the purée, perhaps a single squeeze of lemon or a thin disc of cornichon to lift the salt. The bread needs a real crust because the filling brings no structure of its own, and the brandade is best slightly warm, where the oil stays loose and the cod stays fragrant rather than congealing into a cold block. Warm it too far and the emulsion weeps; serve it too cold and it turns waxy on the tongue.
Variations move along the texture and the aromatics rather than away from the cod. A Nîmes-style purée stays oil-bound and pure, while a softer version folds in potato for a milder, fluffier spread that takes more bread per bite. Some cooks finish it with a rasp of lemon zest or a little chopped parsley, others lay a few black olives alongside for a Provençal edge. Each is a recognizable adjustment of the same emulsified-cod idea. It belongs with the fish sandwiches the catalog groups under Baguette Poisson, and its specific contribution is a cured fish beaten into a spread, so the sandwich's job is to give the emulsion a crust to sit on.