The panino con caponata is a vegetable sandwich whose filling arrives already balanced, because caponata is a finished Sicilian dish before it ever meets bread. Aubergine is fried in cubes until soft and silky, then folded into a sweet-sour base of celery, onion, tomato, capers, and green olives sharpened with vinegar and rounded with a little sugar, the agrodolce tension that defines the whole island's cooking. What makes the sandwich work is that this acid-and-sweet equilibrium is built in the pan, not on the bread: nothing needs to be added to season it, because the caponata has already resolved its own salt, sour, and sweetness into one spoonable thing.
The craft is entirely about moisture, because a wet stew is a hard filling to put between bread. Caponata is drained well and is better eaten a day on, when it has tightened and the flavours have settled, so it holds in the crumb rather than running out of it. It is used at room temperature, never fridge-cold, when the fried aubergine reads softest and the olive oil in it is loose rather than set. The bread is chosen sturdy and open, a crusted roll or a length of Sicilian sesame bread, ideally rubbed with a little oil or lined so the crumb resists the vinegar for the time it takes to eat. The classic build adds nothing, on the logic that a dish carrying its own acid, salt, and sweetness is complete; where anything joins it is a foil that does not fight the agrodolce, a slice of caciocavallo or a few leaves of basil, never a second acid or a cured meat that would crowd an already loud filling.
The variations are the regional caponata itself spooned into the same roll: the Palermo version with pine nuts and raisins leaning sweeter, the Catania reading with peppers, the seafood caponata that adds octopus or tuna. There is also the plate version, the same dish served as a starter or a side with no bread at all, which is a different thing entirely. Each is a different cooked vegetable preparation given a loaf, and each deserves its own article rather than being crowded in here.