The panino con tuma is defined by a cheese that has been deliberately left unfinished. Tuma is the youngest stage of Sicilian sheep's-cheese making, the fresh curd before any salt is added and well before it ages into pecorino: pale, soft, faintly sweet, and almost without character of its own. That blandness is the whole point of the sandwich rather than a flaw in it. Where most Italian cheese panini lean on a wheel at its assertive peak, this one leans on the opposite, a cheese so mild and milky that it works as a quiet, cooling base whose job is to take seasoning rather than provide it. The bread and what little is added around it carry the flavour; the tuma carries the texture and the calm.
The craft is matching a soft, wet, unsalted cheese to bread and to seasoning, because on its own it would read as nearly nothing. Tuma is sliced thick rather than shaved, since its appeal is the yielding, slightly squeaky bite of fresh curd and a thin shaving would disappear entirely. It needs salt added at the table, a few grains and a thread of olive oil being the classic minimal dressing, and it rewards a crack of black pepper or a leaf of something sharp to give the mildness a point of contrast. The bread is plain and fresh, and the sandwich is built and eaten soon, because a fresh unsalted cheese gives up moisture quickly and a roll left to stand will go damp under it. Simplicity here is not laziness; it is the only way to let a deliberately blank cheese be the subject.
The variations are about what small thing is allowed to season the blankness: the version with oil, salt, and pepper alone, the one with a sharp leaf or a few olives, the build where a little cured meat supplies the salt the cheese lacks. Each is a different seasoning of the same fresh curd, and each deserves its own article rather than being crowded in here.