Pecorino toscano is the gentle one, and that gentleness is what makes the panino work the way it does. Where the Lazio cheese is a salt bomb and the aged Sardinian one is granular and sharp, the Tuscan sheep's-milk cheese is soft to semi-firm, mild, grassy, and faintly nutty, a cheese you can eat in slabs rather than shards. It comes young and barely set, semi-stagionato and springy, or stagionato and firmer but still rounded, never aggressive. The defining fact is that this is a cheese with enough restraint to be the bulk of the sandwich rather than its accent: it is sliced thick, layered generously, and asked to lead, which almost no other pecorino in this catalog can do without something blunting it.
The craft is letting a mild cheese stay the subject while keeping it from going bland. The fresh and semi-stagionato forms are soft enough to cut into broad, supple slices that fold into the bread and stay yielding; the stagionato is firmer and cut a touch thinner but still substantial. The bread is the regional signature here: the unsalted Tuscan pane sciocco, whose blankness was made for exactly this, a loaf with no salt of its own so a low-salt cheese is not flattened by it. The pairing is the cheese's own register, quiet things that lift without overpowering: a thread of chestnut honey, a few drops of oil, or simply the bread. The discipline is addition by a single small thing, never a stack.
The variations are Tuscan and seasonal, each leaning the mild base in one direction. The young cheese turns up against fresh broad beans in spring and against honey or a hard pear when something sweet is wanted, and the firmer stagionato takes a salume better than the soft form does. Each is the same gentle Tuscan wheel met by one regional partner, and each deserves its own article rather than being crowded in here.