Osso Buco, or Ossobuco is a veal shank cross-cut so that the marrow is exposed. The slices of shank are braised with vegetables, white wine and broth. It is often garnished with gremolata (parsley, lemon zest, and garlic) and traditionally served with either risotto alla milanese (rice with saffron, often with added bone marrow) as shown in the photo, or polenta. It is sometimes advertised as a “piatto unico” meaning you wouldn’t be expected to order a primo piatto before it.
Around Siena, you might find the dish spiked with tarragon, which is a local herb in that part of southern Tuscany.
The picture is of the osso buco were were served at the Albergo Ristorante Sicomoro, which is a bike hotel in the Lunigiana with a great restaurant.