In most Genoese sandwiches pesto is a condiment. Here it is the whole filling, and that changes what the bread has to survive. Pesto genovese is raw: Genovese basil, pine nuts, garlic, Parmigiano-Reggiano and a sharper aged pecorino, and Ligurian olive oil, pounded together without heat so the basil never bruises dark. Spread thick on bread with nothing else on it, the sauce is asked to do two jobs at once, to be the flavour of the sandwich and to be the thing that keeps the sandwich from going to paste. The oil and cheese in a properly emulsified pesto form a film that sits on the crumb rather than sinking straight through it, which is why this works at all and why a thin, watery pesto fails it completely.
The craft is the emulsion and the bread that carries it. A pesto loose with oil bleeds into the crumb and the bottom slice gives way before the sandwich is finished; a tight one, the cheese and oil bound stiff against the basil, behaves like a structural layer and waterproofs the crumb the way butter does on a jambon-beurre. The bread is chosen with a firm crust and a close crumb so the sauce has something to grip, and it is left plain because a strong country loaf would only argue with the garlic and the pecorino. Assembly is fast and the sandwich is eaten soon: pesto holds its green for an hour or two, then the basil oxidises at the cut edge and the bright herb edge dulls toward grass.
Liguria spreads pesto into many things, and each is its own argument rather than a version of this one. There is the focaccia split and brushed with pesto instead of filled with it, the pesto laid over warm potato and green bean inside the bread the way it is over trofie on the plate, the lighter Provençal-leaning hands that drop the pecorino, and the modern build that adds prosciutto crudo or stracciatella and turns a sauce sandwich back into a filling sandwich. Each is the same raw green paste asked to behave differently against the crumb, and each deserves its own article rather than being crowded in here.