The panino con verdure grigliate works only because the vegetables are grilled before they ever meet the bread, and that is the rule the whole sandwich turns on. Raw courgette, aubergine, and peppers weep water into a crumb and collapse into a damp salad in the hand; the same vegetables taken over a flame have had their moisture driven off and replaced with char, so they behave like a developed filling instead of a heap of slices. Char, not water, is the operative distinction. The grill concentrates what the vegetables have and adds a smoky edge they cannot get any other way, and that is what makes them substantial enough to be the body of a sandwich rather than a garnish in one.
The craft is in the grilling and the dressing, and almost none of it happens at assembly. The vegetables are sliced lengthways, salted to pull out water, and grilled hard enough to take real colour and a bittersweet smoky edge, then drained so they season the bread rather than flood it. They are dressed warm with olive oil and a little vinegar or lemon, often with garlic and torn basil, the acid keeping the oil from sitting heavy and the salt from going flat. Because the vegetables carry no fat or salt of their own beyond what is added, the seasoning has to be deliberate and generous. A spread of pesto or a soft cheese is the usual binder, waterproofing the crumb and giving the loose filling something to hold to where a meat would. It is good at room temperature and often better an hour after building than a minute, the marinade settling into the bread rather than soaking it.
The variations are about which vegetable leads and what binds them: the aubergine-forward build, the pepper-weighted one, the version with a creamy stracchino or mozzarella spread through it, the one finished with rocket for a peppery top note. Each is a different cooked vegetable on the same bread, and each deserves its own article rather than being crowded in here.