The Sandwich Mogettes is built on the Vendée's white bean. Mogettes are the dried haricot beans of the Vendée, cooked slow and soft until they hold their shape but give completely under any pressure, and the sandwich treats them the way another region treats a soft cheese or a pâté: as the spread that defines the bread rather than a filling tucked inside it. Cooked with a bay leaf and a little garlic, then mashed loose or laid in whole and pressed against the crust, the beans bring a mild, earthy, almost chestnut sweetness and a texture between a purée and a coarse paste. This is the everyday bread context for mogettes. There is also a fouée aux mogettes, the beans tucked into a hollow puffed flatbread pulled hot from a wood oven, but that is a different bread doing a different job; here the beans are spread on a split country loaf or baguette and the structure comes from the crust.
The build works because the bean carries fat and salt without needing much help. Cooked properly the mogettes are rich on their own, often finished with butter or a little of the regional cured pork, so the spread reads as substantial rather than thin, and a smear of good salted butter under it bridges the bean to the wheat the way it does for ham. The constraint is the same restraint that governs a pâté sandwich: the bean is gentle and a loud cheese or a sharp condiment will simply erase it. The successful version stays quiet, the beans warm or at room temperature so the fat stays soft, a slice of jambon de Vendée or a turn of pepper if anything at all. The bread needs a real crust because the filling brings no structure of its own, and the beans need to be warm enough that they spread rather than crumble; cold and stiff they break apart and the sandwich falls to pieces in the hand.
Variations stay inside the Vendée pantry. A few slices of the local cured ham laid over the bean is the common addition; a smear of beurre demi-sel under it is near-standard; some cooks fold in a little of the cooking fat to loosen the spread. The Sandwich Mogettes belongs with the plant-forward builds the catalog groups under Sandwich Végétarien, with the note that its defining ingredient is a regional bean treated as its own condiment, so the sandwich's job is to give it bread and get out of the way.