One vegetable carries the whole thing, and it stands or falls on that vegetable and on salt. The build is almost nothing: a split loaf, a layer of butter or a little olive oil, thick slices of ripe tomato laid down the length, a pinch of salt, a turn of pepper. There is no protein and no sauce hiding a weak component, which is exactly why it is a harder sandwich to do well than it looks. With this little in it, every part of it has to be right.
The logic follows from what the tomato is and is not. A ripe tomato is mostly water and acid with a soft, sweet flesh, so it brings juice and brightness but no fat and no structure of its own. That is why fat sits underneath it, butter or oil, supplying the richness the tomato lacks and giving the crumb a barrier against the water. Salt does the rest of the work: it pulls the tomato's flavour forward and seasons the juice that the slices release. The constraint is the bread and the clock. The crust has to be real and the crumb tight, because tomato water moves into bread fast and a soft loaf turns to paste; even a good loaf has a short window before the slices soak it through. It eats cool, fresh, and best within minutes of assembly, when the tomato is loud and the crust still has bite.
Variations stay close to the same garden. A version with a few leaves of basil and a film of olive oil leans toward the Provençal register; one with a slice of fresh white cheese adds a soft, milky counterweight to the acid; the plainest is tomato, salt, butter, and bread alone. Each holds the ripe tomato as the fixed point and changes only what frames it. The Sandwich aux Tomates belongs with the plant-forward builds the catalog groups under Sandwich Végétarien, the tradition that treats vegetables as the lead rather than the absence of meat. Its specific contribution is a sandwich reduced almost to nothing, where the only insurance is a ripe tomato, enough salt, and bread that still has crust.