The Croque au Thon replaces the ham of the canonical Croque-Monsieur with tinned tuna, usually folded with a spoonful of mayonnaise or crème fraîche before it goes between the bread. The cheese stays, the béchamel stays in the better versions, the toasting method stays. What changes is the protein, and what follows from that change is most of the sandwich. Where the ham version trades on the contrast between gently cured pork, melted cheese, and creamy sauce, the tuna version trades on a softer, more uniform set of flavours: the fish, the mayonnaise, the cheese, and the sauce all sit in roughly the same register.
The sandwich works best when the tuna is genuinely good. Tuna packed in olive oil, broken into large flakes, lightly seasoned with capers or pepper, is what the form rewards. The supermarket version of the same idea, made with tuna packed in water and pre-mixed with too much mayonnaise, tends to produce a damp interior that the bread cannot keep up with. The cheese in the better versions is usually Emmental or Gruyère, and a few cooks reach for a softer cheese like a young Comté or Cantal entre-deux to push the fat content up and balance the tuna's leaner profile. A scattering of cornichon slices or a turn of black pepper on top before broiling helps cut the richness. The format is most often found at brasserie lunch counters and on the cafeteria menu, and rarely at the higher-end café where the canonical ham version is the default.
The variant sits inside a small cluster of fish-forward croques that include the Croque Norvégien with smoked salmon (which traditionally skips the béchamel) and the Croque Méditerranéen with sardine or anchovy. None of these have the cultural weight of the Croque-Monsieur proper, but each represents a legitimate substitution that follows the form's logic: the ham was always a protein vehicle, and the protein can change as long as the structural elements of bread, melted cheese, and creamy binder remain in proportion. The tuna version is the most common of the fish variations, in part because tinned tuna lives in every French pantry, and in part because the price point keeps the sandwich firmly inside the everyday lunch category rather than nudging it toward a more elaborate brasserie offering.