The trapizzino alla picchiapò puts a frugal Roman boiled-beef stew into the pocket, and its plainness is exactly why it needs the bread. The shell is the usual trapizzino: a triangle of pizza-bianca dough baked until the crust crackles and the crumb stays open, slit along the long side and held upright to take a wet scoop. Picchiapò is what Roman households did with the meat left from making broth: boiled beef, already cooked grey and tender for the brodo, pulled or sliced and then stewed again with a soft tangle of onion, tomato, and a little of its own cooking liquid until the bland boiled meat is rebuilt into something savoury and slightly sweet from the onion. It is poor cooking by design, with no browning crust and no rich gelatin, so the dough is doing real structural work and the onion-tomato sauce is the entire flavour argument.
The craft is in the sauce, because the meat brings almost nothing of its own. A good picchiapò cooks the onion slowly until it is sweet and almost dissolved, then lets the tomato cook down with the shredded boiled beef long enough that the meat reabsorbs flavour it lost to the broth pot; the result has to be juicy but tight, never the thin watery braise that runs through a trapizzino crumb in seconds. The shell is baked hard at the cut faces so the sauce sits rather than soaks, and the filling goes in hot, pushed to the closed corner of the triangle so the open top stays dry in the hand. A turn of black pepper and, in some kitchens, a few flakes of chilli keep the sweetness of the onion from going slack. The sloppy build under-cooks the onion, leaves the sauce loose, and overfills the pocket so the whole thing weeps.
The close cousins all start from the same broth pot and each deserves its own article rather than being crowded in here. There is bollito served plain in slices with salsa verde and no bread, the lesso rifatto con le cipolle of the north which is the same boiled-beef-and-onion idea cooked drier as a skillet plate, and the trapizzino with tongue in green sauce that solves the leftover-boiled-meat problem from the opposite, sharper direction. Each is a different answer to a pot of spent broth meat, and each is its own subject.