A Pan Bagnat is a salade niçoise that has been allowed to sit. The components are familiar, including tuna, anchovy, hard-boiled egg, olives, tomato, raw onion, and sometimes a leaf of basil, but instead of arriving on a plate they are packed into a round country loaf that has been split, doused with olive oil and a touch of red-wine vinegar, weighted, and set aside for an hour or longer. By the time the sandwich is eaten the oil has soaked into the crumb, the vegetables have given up some of their water, and the bread is no longer crusty in any conventional sense. The name is Niçois for "bathed bread," which describes the technique exactly.
The bread matters more than visitors expect. The traditional loaf is a pain bouclé from a Niçois bakery, round, slightly tart, with a sturdy crumb that can absorb the dressing without disintegrating. Substituting a baguette is the most common mistake outside the region, and produces a sandwich that wets through within minutes. The other immovable rule is that the components are raw or pre-cooked but never warm. This is a beach sandwich, a summer-lunch sandwich, a sandwich whose entire character depends on its components having marinated together rather than been freshly arranged.
Niçois cooks have firm opinions about what does not belong in a Pan Bagnat, and the list is informative. No cooked vegetables: no boiled potatoes, no blanched green beans, however much they appear in plated salade niçoise. No mayonnaise-bound tuna. The tuna goes in plain, broken up over the bread, where the olive oil from the dressing carries the seasoning. No lettuce, which would wilt under the weight of an hour's rest. The orthodoxy is regional and a little proud, but it is also coherent. Every excluded ingredient is excluded because it would not survive the soaking. Modern variations swap the tuna for capers and mozzarella, or push toward more peppers in summer when the tomatoes are loud enough to carry the sandwich on their own. The Niçois flatbread cousins, including Pissaladière and its socca neighbors, show a different way the same Mediterranean palette gets handled in the same town.
The Pan Bagnat is one of the few French sandwiches that genuinely benefits from being made in advance, and one of the few that travels well in summer. Wrapped tightly and kept cool, it improves over the first two or three hours. That property is what makes it a beach lunch in Nice, a market-day staple along the Côte d'Azur, and one of the cleanest examples in the French repertoire of a sandwich whose technique is patience.