The garlic-parsley-lemon peel finish is called "gremolata" and is classic. The hollow bone (osso buco) of the veal shanks gives this dish its name. MELT THE BUTTER in a large casserole. Add the sliced onion and saute over medium heat until soft and just beginning to color (about 10 minutes). With a slotted spoon, remove the onion to a plate and press it against the plate to extract its juices and let them run back into the pan. Discard the onion. Dry the veal pieces with paper towels. Flour them lightly and brown them over medium heat in the onion-flavored butter. Salt and pepper the pieces, raise heat to medium-high and pour on half the wine. After a moment, turn the pieces of veal over, salt and pepper the other sides. Pour on the rest of the wine when the first half has evaporated. When all the wine has evaporated, arrange the veal pieces so that the wide marrow opening is up. Add the diluted tomato paste, cover, and cook over low heat about 1 1/2 hours, or until the veal is tender. During that time, move the pieces around to prevent them from sticking, and baste them occasionally with their cooking juices, using a bulb baster, but don't turn them over lest the marrow dissolve and run out. (You can add a tablespoon or 2 of warm water if the sauce seems to be drying out--but the veal will probably exude plenty of its own juices after the first half hour of cooking.) While the veal is cooking, finely chop together the garlic, parsley and lemon. Five minutes before serving, stir it into the casserole. Classically, this dish is served with risotto Milanese, made with saffron and Parmesan cheese, but any simple rice or pasta preparation will do.