The Sandwich Brandade de Morue is the potato-bound reading of a cured-fish spread. Morue is salt cod, soaked to draw the salt down and then poached and flaked; in the Languedoc and Provence the brandade de morue beats that cod with olive oil, warm milk, garlic, and a measure of mashed potato until the whole thing comes together as a thick, pale, scoopable purée. The potato is the defining choice here: it stretches the cod, rounds its salt, and gives the filling a softer, fluffier body than an oil-only version. Spread into bread, that purée is what makes this a sandwich rather than a fish fillet between slices.
The craft is in how the potato changes the filling's behavior. A potato-bound brandade is creamier and less aggressively salty than the pure emulsion, so it takes more of itself per bite and forgives a slightly larger portion without overwhelming the bread. It still carries its own fat from the oil and the milk, so butter underneath is redundant. It also stays cohesive: the starch helps the purée hold a shape and grip the crumb instead of sliding. The constraint is heat and moisture. Brandade de morue is best gently warm, where the oil stays loose and the garlic stays fragrant; pushed hot it sweats and slackens, and held cold it stiffens into a paste that drags. The bread needs a firm crust to stand up to a soft, slightly oily filling that brings no structure of its own, and the sandwich is best within a few minutes of being filled, before the warm purée begins to steam the crumb soft.
Variations sit on the potato dial and the aromatics. More potato gives a milder, more bread-friendly spread closer to a fish brandade gratin scooped cold; less potato pulls it back toward a sharper, saltier, oil-driven purée. A clove more garlic, a little parsley, a few drops of lemon, or a few olives laid alongside are the usual moves, each a recognizable turn on the same cod-and-potato base. It belongs with the fish sandwiches the catalog groups under Baguette Poisson, and its specific contribution is a cured cod softened with potato into a spread mild enough to fill a sandwich generously.