1. Stud onion halves with cloves.
2. Place thyme, parsley, and bay leaf on a double layer of cheesecloth. Gather edges of cheesecloth together; tie securely.
3. Place onion halves, cheesecloth bag, White Veal Stock, and next 4 ingredients (through chuck roast) in an 8-quart stockpot; bring to a boil. Reduce heat, and simmer, uncovered, 3 1/2 hours.
4. Tie twine around leek halves to secure. Add leek halves, carrots, and turnips to pan; simmer 1 1/2 hours or until vegetables and meat are tender. Strain mixture through a cheesecloth-lined colander into a large bowl; discard onion and cheesecloth bag. Place 2 large zip-top plastic bags inside 2 large bowls. Carefully pour stock mixture into bags; let stand 10 minutes (fat will rise to the top). Seal bags. Working with 1 bag at a time, carefully snip off 1 bottom corner of each bag. Drain stock mixture into pan, stopping before fat layer reaches opening; discard fat. Stir in salt.
5. Shred roast with 2 forks. Remove meat from shank and rib bones; discard bones, fat, and gristle. Add meat to stock mixture. Serve with mustard and cornichons.
Wine note: Although Pot-au-Feu is chock-full of meat, this version is not as heavy as others. Therefore, I like to serve a lighter-style cabernet, one that's medium bodied rather than full bodied but still has exquisite flavors. The elegant cabernets of Australia's Margaret River region fill the bill nicely. One of the best—and a wine that will certainly elevate this dish to entertaining status—is Leeuwin Estate 2002 Art Series Cabernet Sauvignon, about $45 —Karen MacNeil