Pecorino romano is not a melting cheese and was never meant to be a slicing one, which is the first thing to understand about putting it in bread. This is the hard, brittle, aggressively salty sheep's-milk cheese of Lazio, the one that finishes a cacio e pepe or an amatriciana, and on a panino it behaves the same way it does on pasta: in shards. A wedge is cracked rather than carved, broken along its grain into uneven splinters that are scattered through a plain roll, and the sandwich is essentially a way to eat grating cheese by hand. The salt is the defining fact. Where a gentler cheese asks the bread to stay quiet, pecorino romano is so forceful that the bread is there mostly to absorb and dilute it, a structural buffer as much as a vehicle.
The craft is managing that salt without softening what makes the cheese itself. The shards are kept coarse and irregular so each bite lands a sharp, crystalline hit rather than a uniform layer, and the bread is chosen plain and absorbent for the same reason a cacio e pepe needs starchy water: something has to round the edge. A thread of olive oil is the usual and almost only addition, slicking the splinters so they cling to the crumb instead of scattering out of the roll, and occasionally a few cracked peppercorns echo the pasta it came from. Bread, salt cheese, oil, pepper: the whole construction is restraint enforced by necessity, because almost anything else added at this salinity would simply lose. It is eaten as a fierce, fast bite, not a leisurely one.
The variations stay close to the Roman table that produced it. There is the version that leans fully into its origins with cracked black pepper worked through, a cacio e pepe logic carried into bread, and the rougher field build where the shards meet raw broad beans or a hard pear to blunt the salt. These follow the same shard-and-buffer principle with one thing changed, and each deserves its own article rather than being crowded in here.