The Sandwich Thon-Mayonnaise is the plainest member of the tuna family: tinned tuna, mayonnaise, bread, and as a rule nothing else. It is the everyday version, the one with no vegetables layered over it and no salad ambitions, the default fish sandwich pulled from a boulangerie case or made in under a minute at a kitchen counter. The tuna is flaked, folded with enough mayonnaise to bind it, sometimes loosened with a little lemon or a few chopped cornichons, and packed into a split baguette. There is no cooking and almost no technique. The whole sandwich is decided by the quality of the tin and the ratio of bind to fish.
The logic is rescue from blandness. Tuna out of a can is mild and, if it is the kind packed in water and over-drained, dry, so the mayonnaise is doing structural work: it binds the loose flakes into a cohesive layer and restores the fat that gives the filling body. Too little and the sandwich is dry and dull; too much and it slumps into paste and the bread goes wet. A little acid pulls it back into focus, because tuna mayonnaise without lemon or a sharp pickle reads flat within a few bites. This is the pared-down version on purpose: where the Sandwich au Thon treats the tin and its additions as a place to vary, the thon-mayonnaise is the baseline both of them start from, the bind-and-fish core with the variations stripped off.
The bread carries what the soft filling cannot. A baguette with a firm crust and an open crumb holds the dressed tuna without going slack, and the filling is mounded loose rather than packed dense so it does not compress into a brick. There is no warm component, and the sandwich is best made close to when it is eaten: tuna mayonnaise sitting in bread soaks the crumb and dulls within the hour.
Variations are small and stay close to the bind. Oil-packed tuna instead of water-packed makes a rounder base and needs less mayonnaise, which is the single change that most improves it; a spoon of crème fraîche worked in lightens and sharpens it; a little chopped cornichon or caper adds the bite the plain version lacks. Each keeps the two-ingredient core fixed and adjusts only the fat or the acid. It belongs with the fish sandwiches the catalog groups under Baguette Poisson, and its specific contribution is plainness: the irreducible tuna sandwich, fish and bind and bread, whose quality is set almost entirely by the tin.