The Sandwich Niçois is the salade niçoise rebuilt for the hand rather than the fork, and the thing that separates it from its closest relative is what the bread is asked to do. Where the Pan Bagnat takes the same Niçois palette and lets a round country loaf soak in oil and vinegar until it goes soft, the Sandwich Niçois keeps its bread intact: a crusted length or a split roll that stays a sandwich through the last bite. The filling is the familiar Nice register, including tinned tuna in olive oil, anchovy, hard-boiled egg, small black olives, ripe tomato, raw red pepper, a few leaves of basil, dressed with olive oil and a touch of red-wine vinegar. The same ingredients, a different contract with the crumb.
That distinction is the whole craft of it. Because the bread is not meant to absorb the dressing and collapse, the assembly is built to be eaten close to when it is made rather than left to marinate. The tuna goes in plain, broken over the bread so the oil it carries seasons the crumb directly; the anchovy does the salting; the egg and the tomato supply the soft and the wet against a crust that still has bite. A good one is generous with the oil but stops short of saturation, which is the line between a sandwich that holds and one that has quietly become a Pan Bagnat by accident. The components are raw or already cooked, never warm, which keeps it firmly in the register of a summer lunch and a thing you carry to eat somewhere with a view.
Variations stay inside the Nice pantry rather than wandering off it. A vegetarian build trades the tuna for more pepper and tomato, leans on capers where the anchovy was, and lets the produce carry the sandwich when the tomatoes are loud enough to do it. Some versions add a smear of tapenade to the crust, others fold in thin raw fennel or radish for crunch, and the proportions shift toward whichever vegetable the market favored that morning. It belongs with the place-named sandwiches the catalog groups under Regional Specialty Sandwiches, and its specific contribution is to show that the Niçois idea survives without the soak: keep the bread honest, and the salad becomes a sandwich that travels on its crust rather than in spite of it.