The tuna Niçoise sandwich is the salade niçoise read as a filling, and what defines it is that the tuna is no longer the whole story. Tuna, hard-boiled egg, black olives, and green beans go onto bread together, with the components kept distinct rather than mashed into a bound paste, so the sandwich eats as a composed salad between two slices instead of a tuna mayonnaise with extras. The egg and the olive are doing as much identifying work as the fish: the brine of the olive, the richness of the yolk, and the snap of the bean are the flavours that tell you this is a Niçoise reading and not a tuna salad sandwich that happens to have an egg in it.
The craft is composition and moisture control across several wet things at once. Each element brings its own liquid problem: the tomato weeps, the egg slumps if it is overcooked and chopped too fine, the beans hold water if they are not properly drained, and the tuna sheds oil. The standard answers are to keep the egg in firm slices or wedges rather than a wet mince, to blanch and cool and dry the green beans so they stay snappy rather than going limp and watery, to deseed the tomato, and to dress the assembly lightly with oil and a sharp vinaigrette rather than binding it in mayonnaise, because mayonnaise would flatten the brisk, briny character that is the point. The bread needs more structure than a soft white slice gives, a baguette or a sturdy roll that can hold a loose, multi-part, lightly dressed filling without collapsing, and it is dressed and pressed slightly so the components settle into each other rather than spilling at the first bite.
The variations track the salad it came from rather than the tuna shelf. A version that adds anchovy alongside the tuna deepens the brine; one built on a split baguette and pressed so the bread compacts around the filling reads closer to a pan-bagnat; a plainer take drops the bean and leans on egg, olive, and tomato. The cold bound tuna sandwich and the grilled tuna melt are separate forms entirely. Each deserves its own article rather than being crowded in here.