The Pissaladière is, depending on who you ask, a tart, a sandwich, or a Mediterranean cousin of focaccia that argues for its own category. A bread base (sometimes a pizza-style dough, sometimes the local pain de campagne) is covered with onions slow-cooked until they're nearly jammy, dotted with cured anchovy fillets, and studded with small black Niçois olives. It bakes flat, comes out warm, and is cut into squares for the bar counter.
The name comes from pissalat, the Niçois fish paste of fermented anchovies and herbs that originally coated the dough before the cooked onions were layered on. The pissalat itself has mostly disappeared from the modern version — most bakers now just dot anchovy fillets on top — but the principle is the same: a few intensely-flavoured ingredients carry the dish, and the bread is there to hold them in place.
It is technically not a sandwich in the strict bread-on-bread sense, but it shows up in this list because in Nice it's eaten by hand, between meals, off a paper square, on the way somewhere — which is the working definition of a sandwich. The Socca — a chickpea-flour pancake from the same Niçois tradition, eaten warm with cracked pepper — falls into the same category, as does the Fougasse d'Aigues-Mortes, the herb-and-olive bread that travelers eat on the way through the Camargue. The variations along the Côte d'Azur are minor: more olives in Antibes, less anchovy in Cannes, occasionally a few rounds of tomato in summer. The principle holds: onion, anchovy, olive, bread, salt.