Primo sale is a Sicilian sheep's-milk cheese caught at the first salting, which is the whole identity of the panino built on it. It is barely a cheese yet: the curd is pressed, given its first light salt, and eaten within days, while it is still white, soft, springy and milky, before any rind or sharpness develops. That youth is the point. Where an aged pecorino is loud and demands restraint around it, primo sale is mild and delicate, and the sandwich is an exercise in not drowning a quiet thing. A few slices on the right bread, with almost nothing else, and the milkiness is allowed to be the entire flavour.
The craft is handling a wet, fragile cheese and keeping the bread out of its way. Primo sale carries moisture and crumbles if pushed, so it is cut into thick, careful slices rather than shaved, and laid in without pressing so it holds its springy texture instead of weeping into the crumb. Because it is so mild, anything assertive buries it, so the build stays spare: good Sicilian olive oil, a turn of black pepper, sometimes a few fresh fava beans in spring, which is the classic Sicilian pairing and adds grassy bite without overpowering. The bread is taken plain and fresh, a soft sesame roll or a close country loaf, chosen to carry rather than compete, and the sandwich is assembled and eaten soon, because a fresh cheese this young is at its best before the fridge dulls it.
Sicily and the wider South make several of these very young sheep and goat cheeses, and each is its own subject rather than a version of this one. There is the tuma, the unsalted curd one stage younger still, the ricotta salata taken the other direction toward firm and salty, the fresh cacioricotta, and the spring hand that pairs primo sale with fava and a little mint. Each is a different fresh wheel given a Sicilian bread, and each deserves its own article rather than being crowded in here.