The Pissaladière Sandwich is what happens when the Niçois onion tart is treated as bread rather than as a tart. The base is a yeasted dough, closer to a thick focaccia than to pastry, spread with onions cooked down slowly until they are soft, sweet, and nearly jammy, then laid with cured anchovy fillets and small black Niçois olives. To eat it as a sandwich you do not add a filling: you fold a square in half, or you split the dough base and use the cooked onion layer as the savory interior. The onions are the filling. The bread is also the bread. It is one of the few sandwiches in the French repertoire where the topping and the structure are the same object.
The logic is the logic of a base that is already fully seasoned. Those slow-cooked onions carry the salt, the sweetness, and the weight; the anchovy supplies a sharp cured edge; the olive a bitter punctuation. Folded, the yeasted base has enough body to hold the soft onion without tearing through, and enough chew to stand up to oil rather than going to paste. This is why nothing else is added: a slice of ham or a spread of cheese would only bury a layer that was built to be the whole point. In Nice it is sold warm by the square, eaten off paper, by hand, between meals, on the way somewhere, which is the working definition of a sandwich even though there is technically only one piece of bread involved.
Variations are small and mostly about the ratio. More olive toward Antibes, a lighter hand with anchovy toward Cannes, occasionally a few rounds of tomato in summer when they are good enough to carry it. The defining frame holds: onion, anchovy, olive, yeasted base, salt, eaten warm and folded. The Pissaladière Sandwich belongs with the southern bread snacks the catalog groups under Pissaladière & Niçois Bread Snacks, the Mediterranean flatbreads the French sandwich tradition handles apart from the baguette. Its specific contribution is the collapse of the usual distinction: a sandwich where there is no separate filling because the seasoned surface and the bread are one and the same.