The panino con ossobuco is a Milanese braise asked to behave inside bread, and the gap between those two things is the whole sandwich. Ossobuco is a cross-cut veal shank braised long and slow with wine, stock, and aromatics until the meat slips off the bone and the marrow in the central hole turns soft and spoonable, finished with gremolata, the raw mince of lemon zest, garlic, and parsley. It is a plated dish eaten with a fork and a small spoon for the marrow, deliberately wet, which is exactly the problem this panino exists to solve: how to carry a loose, rich braise and its marrow between two pieces of bread without the bread giving way before the last bite.
The craft is making the shank hold and choosing a loaf that can take what it cannot fully tame. The veal is pulled off the bone and the marrow worked back through it, the braising liquid reduced hard so the meat goes in coated rather than swimming, because the loosest liquid is what destroys the crumb. The roll is sturdy and crusted, often toasted on the cut faces so it resists the fat and the sauce for longer, and the portion is kept controlled since an overfilled one fails at the seam on the first bite. The gremolata is the one essential addition and not a garnish: the raw lemon zest and garlic cut straight through the marrow's richness, doing the job a pickle does elsewhere, so the sandwich does not read as one heavy note. It is eaten warm, when the fat is liquid and forgiving, never cold, when it sets and the bread turns heavy.
The variations are the leftover-and-stall tradition rather than codified recipes: the chalkboard build at a Lombard trattoria, the festival-stand version where last night's shank becomes today's roll, the related Milanese braises that meet bread the same way. Each is a plated dish given a handle on its own terms, and each deserves its own article rather than being crowded in here.