This sandwich is the same thin, oily Genoese focaccia as its cheese cousin, split and filled, but the filling changes what the bread is for. Where a soft cheese meets the oil and salt with something cool and milky, prosciutto crudo meets it with cure and fat, and the two readings of salt stack rather than balance. Focaccia genovese is the slack, dimpled kind, its surface pooled with olive oil and a salty brine in the finger holes, baked thin so the crumb stays slick and the top goes gold. Lay a few folds of sweet, fat-marbled prosciutto into that and the bread's oil glosses the meat from below while the cure carries the bite, a sandwich in which both halves are salty on purpose and the pleasure is in not flinching from it.
The craft is the slice of the meat and the restraint to add nothing that would muddle two clear flavours. The prosciutto is cut paper-thin and laid in loose, airy folds rather than flat sheets, so the salt arrives in a draped texture and the fat has room to soften against bread that still holds a little warmth. A thicker cut would read as a slab and overwhelm a focaccia that is already assertive; the thin fold lets the oil of the bread and the fat of the meat meet as one slick note instead of two heavy ones. Nothing else is needed and nothing else is added, because the whole point is the dialogue between an oiled, salted bread and a cured, salted meat, and a sauce or a leaf would only get between them. It is assembled fast and eaten while the focaccia is at its best, before the crumb tires under its own oil.
The variations stay Ligurian and stay spare: the version with a milder cured meat in place of the prosciutto, the one finished with a few leaves of rocket against the salt, the taller oilier focaccia barese used in the South under the same instinct. Those are different breads making a related argument. Each of them deserves its own article rather than being crowded in here.