The panino con musso is a Palermo street sandwich built around stewed donkey meat, and the meat is the entire reason it exists. Musso is donkey, simmered long and slow with tomato, onion, and herbs until the lean, dark, slightly sweet flesh turns tender and the cooking liquid reduces to something rich and gelatinous. It is a stall dish, sold from the same kind of cart that handles spleen and offal, ladled hot into a soft roll so a slow braise becomes something eaten standing in the street. The defining fact is that the filling is a finished stew, not a cured or sliced ingredient, and the bread is a carrier for it rather than a structural partner with its own voice.
The craft is the braise and a roll that can take it wet. The donkey is cooked down for hours so the connective tissue breaks and the meat pulls apart under a fork, the sauce reducing until it coats rather than runs, because a loose, watery ladle would soak the crumb to paste before the sandwich is finished. The roll is the plain, soft, faintly sweet Palermo bread, the same vehicle the street uses for everything hot and oily, its only job to hold a rich filling upright for the few minutes it takes to eat. Nothing sharp is added beyond what the pot already carries: the stew arrives seasoned through its depth, and a strong condiment would only argue with a meat that is already the whole statement. It is served the moment it is handed over, when the sauce is hot and the bread has not yet gone slack.
The variations are the stall itself rather than codified recipes: the version eaten as a bowl with the roll on the side to dunk, the one cut with more chilli, the related donkey and horse stews the same carts handle the same way. Each is a different braise given the same roll and the same handle, and each deserves its own article rather than being crowded in here.