The trapizzino alla parmigiana takes a baked vegetable casserole and treats it as a pocket filling, and the bread and the melanzane are built to lean on each other. The shell is the standard trapizzino triangle: pizza-bianca dough baked to a crackling exterior with an open, springy crumb, slit along the long edge and stood up so it can carry something soft and oily without slumping. The filling is eggplant parmigiana scooped out of its dish: slices of melanzane fried or griddled until collapsed and sweet, layered with tomato sugo, basil, and a generous amount of melted mozzarella and grated pecorino or parmesan, then baked until the whole thing sets into a unit you can spoon. It is the only purely vegetable build among the Roman pocket fillings, and the bread matters more here because the parmigiana has no bone, no gelatin, and no bite of its own to give it structure.
The craft is keeping a dish that is essentially soft and wet from defeating the crumb. A good parmigiana for this use is built drier than the one you serve on a plate: the eggplant is salted and pressed so it does not weep, the sugo is reduced until it holds its shape, and the cheese is enough to bind but not so much that the filling turns to a hot liquid the moment it is scooped. The trapizzino shell is baked hard at the cut faces so the oil from the eggplant sits on the crumb rather than soaking straight through it, and the filling goes in hot and firm, pushed to the sealed point of the triangle. A torn basil leaf and a final dusting of pecorino on top keep the sweetness of the eggplant from going flat. The sloppy version uses a loose, plate-style parmigiana, overfills the pocket, and lets it stand until the bread gives way.
The near relations stay in the southern vegetable kitchen and each deserves its own article rather than being crowded in here. There is the parmigiana served as its own baked plate with no bread, the rolled involtini of eggplant around ricotta that share the sugo and basil but not the casserole structure, and the Sicilian habit of frying the eggplant in thicker rounds for a coarser, oilier layer. Each is the same vegetable argued in a different format, and each is its own subject.