In a 12-qt. pot, bring 4 qt. water to a boil over medium-high heat. Add the beef and thyme; season with 2 tsp. salt. Stick the cloves into the onion and add to the pot. Lower the heat and simmer, skimming, until the meat is very tender, about 2 1/2 hours.
Transfer the beef to a large bowl; discard the onion. Place a fine-mesh strainer over a heatproof bowl and strain the cooking liquid. Return the liquid to the pot. Add the carrots, cabbage and turnip, cover and cook over medium heat until tender, 10 to 12 minutes.
When the beef is cool enough to handle, shred into pieces, discarding the bone and nay large chunks of fat. Return to the pot and heat through. Spoon the beef and vegetables into large, shallow bowls, ladle a little of the broth over them and server. pass the mustard and cornichons, plus more salt and pepper, for garnishing.