The Sandwich Thon-Tomate is the lightest reading of the tuna sandwich: tinned tuna and ripe tomato, and very little standing between them. Where the mayonnaise version is a bound, fat-carried filling and the crudités version is a tuna base under a whole salad, this one strips back to two ingredients in a Mediterranean register. The tuna is flaked, dressed lightly, often with olive oil and lemon rather than a heavy mayonnaise, and layered with thick slices of ripe tomato in a split crusted loaf. A few flakes of salt, sometimes a turn of pepper or a leaf of basil, and the build is done. It is the version that reads of the south coast rather than the deli case.
The logic is the tomato carrying the fish. Tinned tuna is mild and dense; a ripe tomato is sweet, acidic, and full of juice, and it does for this sandwich what a heavy bind does for the plainer one, lifting the tuna and giving it brightness instead of weight. Olive oil rather than mayonnaise keeps the whole thing in a cleaner register, closer to a plate of tomatoes dressed with tinned tuna than to a folded salad. The defining problem is water. A cut tomato sheds liquid the moment it goes in, and a sandwich traps what a plate would drain, so the craft is managing that: the tomato sliced, salted, and left a moment to weep before it is layered, the oil sitting on the crumb as a thin barrier rather than pooling.
The bread has to have a real crust and a fairly tight crumb to resist what the tomato still gives off, which is why it holds on a split baguette or a firm country loaf and turns to paste on anything soft. There is no warm component and no reason to wait. This is a sandwich made close to when it is eaten, the tomato salted a few minutes ahead and the assembly done last.
Variations stay within the same short list. A version with anchovy and olive pushes it toward the southern table and a saltier register; one with a thread of basil and more oil reads greener; the plainest holds to tuna, tomato, oil, and salt and trusts the ripeness of the fruit to carry it. Each keeps the tuna-and-tomato core fixed and changes only what dresses it. It belongs with the fish sandwiches the catalog groups under Baguette Poisson, and its specific contribution is lightness: the tuna sandwich read through a ripe tomato instead of a bind, where the season of the fruit decides whether it works.