Wikipedia
The Pan bagnat ( for bathed or wet bread, also spelled as pan bagnat or pan bagna, and alternatively in French as pain bagnat) is a sandwich that is a specialty of the Provence region of France. It is also a specialty food and street food of Nice, France. The sandwich is composed of pain de campagne, whole wheat bread formed in a circle, although white bread is also sometimes used, around the classic Salade Niçoise, a salad composed mainly of raw vegetables, hard boiled eggs, anchovies and/or tuna, and olive oil (never mayonnaise). Sometimes balsamic vinegar, ground pepper, and salt will also be added. It has historically been prepared as a use for day-old bread.
The pan bagnat is a popular dish in Provence and the region around Nice where it is sold in most bakeries and markets. Pan bagnat and the salade niçoise (salade nissarda), along with ratatouille (La Ratatouia Nissarda in Provençal), socca and pissaladière are strongly linked to the city of Nice, where they have been developed over time out of local ingredients. It is sometimes served as an hors d'oeuvre.