A Sandwich aux Moules turns a pot of steamed mussels into something portable. The mussels are cooked open, shelled, and dressed lightly with oil or a thin mayonnaise, a little lemon, and pepper, then mounded into split bread. It reaches the Atlantic coast, where mussels are an everyday catch and cooking them by the potful is routine.
The whole discipline is moisture and timing. Cooked mussels hold water and go rubbery if they wait or overcook, so the steaming is short and the sandwich is built close to service. The dressing stays thin, there to carry the mussel's salt and a note of acid rather than to mask it. A crusted loaf holds the mound loose so the pieces keep their bite; packed tight, they weep and turn to paste. There is no warm element and no reason to wait. The work was done at the pot; the bread just keeps it cold and intact.
The Sandwich aux Moules sits with the cold-water builds the catalog groups under Baguette Poisson. Its specific contribution is the mussel's sweet-and-briny character, set into bread before it fades.