The Pan Bagnat au Thon is the reading of the sandwich where tuna is the lead and everything else is built to carry it. Tinned tuna in olive oil, broken into coarse flakes rather than mashed, is laid generously across the oiled crumb of a split round country loaf, and the supporting cast of the Niçois sandwich, hard-boiled egg, olives, tomato, raw onion, anchovy, arranges itself around the fish rather than competing with it. The dressing is olive oil and a touch of red-wine vinegar, the loaf is weighted and left to sit, and the tuna's own oil bleeds into the bread alongside the dressing. This is the version where, if you press the cut face, it is tuna you see first.
What makes the tuna-forward build distinct is how the fish behaves under the rest. The tuna goes in plain, never bound with mayonnaise, so during the soak the olive oil from the tin and the oil from the dressing carry the seasoning down into the crumb and the fish stays in identifiable flakes rather than dissolving into a paste. A heavier hand on the tuna means the loaf has to be the sturdy slightly tart round of the tradition: too soft a bread and the weight of the fish plus an hour's rest collapses it. The reward is a denser, more saturated centre than a lighter pan bagnat gives, the kind of sandwich whose oil marks the paper it is wrapped in and which is meant to be eaten with both hands at the beach or the market rather than at a table.
Variations on the tuna reading are mostly questions of how loud the fish is allowed to be against the rest. Some hands push the anchovy back almost to nothing so the tuna stands alone; others keep a single fillet for the salt edge it gives the fish. More tomato in high summer lifts and lightens a tuna-heavy build; more raw onion sharpens it. What does not change is the orthodoxy inherited from the parent: no mayonnaise, no cooked vegetables, the bread soaked rather than crisp, the whole thing improving over its first hours. For the full account of the soak, the bread, and the Niçois rules this version inherits, see Pan Bagnat.