The panino con parmigiana di melanzane is the specifically aubergine build, and the aubergine is what makes it work where other vegetables would not. Slices of aubergine are salted to draw their water, fried until they collapse to something silky and dense, then layered with tomato sauce, mozzarella or fior di latte, grated cheese, and basil and baked until the whole sets. Fried aubergine has a structural advantage no raw or watery vegetable has: it holds its body under heat and pressure and reads as a substantial, almost meaty layer. That collapsed, oil-rich flesh is the entire reason a wet bake survives inside bread at all, and it is what defines this sandwich against the looser vegetable parmigiane.
The craft is the salting, the fry, and the moisture that follows. Salting and pressing the aubergine before frying is not optional: it removes the water that would otherwise weep into the bread and turn the panino to paste within minutes. Fried properly the slices are dense rather than greasy, and they go into a bake that is best rested so the cheese sets and the sauce stops running. The roll is sturdy and often warmed so a firm cut face meets the filling, and the portion is kept measured because an overfilled one fails at the bottom seam. Served warm rather than scalding, the mozzarella binds the aubergine layers into one mass instead of sliding free; the basil is laid in late so it stays bright against the sweet tomato.
The named directions stay close to the Campanian original. There is the Sicilian reading that adds a touch of sugar or a harder grated caciocavallo and reads sweeter and sharper, the version built with smoked provola for a deeper note, and the lighter grilled-not-fried take that reads cleaner but holds less well in bread. The general non-aubergine parmigiana is a separate dish and stands apart. Each of these is its own preparation, and each deserves its own article rather than being crowded in here.