The Sandwich Caprese is built on three ingredients that have to be good or there is nothing else to fall back on: ripe tomato, fresh mozzarella, and basil, the Italian-influenced trio that reaches France as a summer sandwich rather than a plated salad. There is no cured meat carrying it, no cheese with weeks of age behind it, no sauce. The tomato has to be in season, the mozzarella has to be the soft fresh kind rather than the firm cooking sort, and the basil has to be torn rather than chopped so it does not bruise to black. A film of olive oil, a few flakes of salt, sometimes a turn of pepper, and the build is finished.
The defining problem is water. Tomato and fresh mozzarella both shed liquid the moment they are cut, and a sandwich is a closed structure that traps what a salad plate would drain away. The whole craft of the Caprese sandwich is managing that: the tomato sliced and salted and left a moment so it sheds before it goes in rather than after, the mozzarella torn and patted, the oil sitting on the crumb as a thin barrier rather than pooling. The bread has to have a real crust and a tight crumb to resist what still gets through, which is why it works on a split crusted loaf or a ciabatta and turns to paste on anything soft. There is no heat and no reason to wait. This is a sandwich made close to when it is eaten, the salting done a few minutes ahead, the assembly done last.
Variations stay within the same short list of ingredients. A version with a thread of pesto pushes the basil further and adds its own oil; one with a stripe of balsamic reduction trades some of the clean dairy register for sweetness and acid; the plainest is tomato, mozzarella, basil, oil, and salt, trusting the three components to carry it with nothing added. Each holds the tomato-mozzarella-basil core fixed and changes only what dresses it. The Sandwich Caprese belongs with the plant-forward builds the catalog groups under Sandwich Végétarien. Its specific contribution is fragility as a design constraint: a sandwich whose quality is decided by the ripeness of the tomato and how well the water was managed before the lid went on.