How to Make It
Season the veal shanks generously with salt and pepper. In a very large enameled cast-iron casserole set over a heat diffuser (if you have one), melt the butter in the oil. Stir in the turmeric, then add the veal shanks. Cook over moderate heat, turning once, until lightly browned on both sides, about 6 minutes total. Transfer the veal to a plate.
Add the onions and a generous pinch of salt to the casserole and cook over moderate heat, stirring occasionally, until barely softened, about 5 minutes. Add 4 cups of water; bring to a boil over high heat. Nestle the veal shanks and the spice sachet in the braising liquid. Cover the casserole and braise over low heat until the veal is just tender, about 1 hour and 30 minutes. Uncover the casserole and cover the shanks with a large piece of parchment. Cover the casserole and continue to braise over low heat until the veal is very tender, about 45 minutes longer.
Using a slotted spoon, transfer the veal shanks and onions to a platter and tent with foil. Boil the braising liquid over high heat until slightly reduced but still light in color, about 10 minutes. Discard the strings from the shanks and spoon some of the braising liquid on top. Serve.