The panino con cassoeula is a Milanese winter braise asked to behave inside bread, and the tension between those two things is the whole sandwich. Cassoeula is a long, fatty Lombard stew of the humbler pork cuts, ribs, rind, trotter, the verzini sausages, cooked down with Savoy cabbage until the cabbage collapses into the pork fat and the whole pot turns rich, soft, and deliberately wet. It is a plated Sunday dish with a spoon, not a sandwich, which is precisely the problem this panino exists to solve: how to carry a loose, gelatinous braise between two pieces of bread without the bread surrendering before the last bite.
The craft is making the stew hold and choosing a loaf that can take what it cannot fully tame. The cassoeula is drained hard or reduced down so the solid pork and the wilted cabbage go in without their loosest liquid; the meat is pulled off the bones and the rind chopped small so each bite is structured rather than sliding. The bread is a sturdy crusted roll, often toasted on the cut faces so the crumb resists the fat for longer, and the portion is kept controlled because an overfilled one fails at the seam on the first bite. Nothing sharp is added: the dish already carries its own slow-cooked depth and its own salt, and a pickle or a strong cheese would only fight the cabbage. It is eaten warm, when the fat is liquid and forgiving, never cold, when it sets and the bread goes heavy.
The variations are essentially the leftover-and-festival tradition rather than codified recipes. There is the chalkboard build at a Lombard trattoria and the festival-stand version where last night's pot becomes today's roll, and there are the related Lombard braises that meet bread the same way. Each of those is a plated dish given a handle on its own terms, and each deserves its own article rather than being crowded in here.