Cook the mussel right and use it soon, or this shellfish build dies on both counts. Mussels are steamed open, pulled from their shells, and bound lightly, sometimes with a thin mayonnaise, sometimes just oil, lemon, and pepper, before being mounded into split bread. It reaches the Nord and Atlantic coasts, where mussels are a staple of the catch and cooking a pot of them is routine kitchen work. The mussel is sweet and saline at once, and the sandwich is designed to keep that intact.
The craft is about moisture and timing. A cooked mussel holds water and turns rubbery if it sits or is overcooked, so the steaming is brief and the assembly happens close to service. The dressing stays thin because the point is to carry the mussel's own salt and a note of acid, not to coat it. The bread takes a real crust and holds the mound loose, since mussels packed tight weep and the texture collapses; loosely piled, each one keeps its bite. There is no warm component to the finished sandwich and no reason to wait. The work belongs to whoever ran the pot; the bread keeps it cold and whole to the bite.
Variations stay coastal. A spoon of rémoulade or a little shallot sharpens it; cold mussels left in a splash of their cooking liquor with parsley keep the marinière note; the same bread takes shrimp or cockles when the mussels are not in. The Sandwich aux Moules sits with the cold-water builds the catalog groups under Baguette Poisson. Its specific contribution is the mussel's double character, sweet and briny together, set into bread before either fades.