Pierce each whole onion with a clove. Place onions, 2 leeks, 2 carrots, 3 celery ribs, and garlic in a large pot. Set roast on top of the vegetables. Wrap parsley, thyme, and bay leaves in a moistened piece of cheesecloth and tie into a bundle. Add the cheesecloth bundle, salt, and peppercorns to the pot.

Pour beef stock over the roast and vegetables. Bring to a boil. Reduce heat to low. Cover partially and simmer, skimming occasionally, until roast is very tender, about 2 1/2 hours.

Transfer roast to a large bowl and cover. Strain broth, discarding solids, and return it to the pot. Boil over high heat until reduced to 8 cups, about 45 minutes. Skim off the fat.

Stir remaining 2 leeks, 2 carrots, and 3 ribs celery into the reduced stock. Add turnips, rutabagas, and parsnips. Cover and simmer over low heat until vegetables are just tender, about 30 minutes. Add potatoes. Cover and simmer until potatoes are tender, about 40 minutes more. Add sausage; simmer until heated through, 5 to 10 minutes. Season the broth with salt and pepper.

Slice the roast across the grain into 6 to 8 slices. Ladle the broth into shallow bowls. Plate roast, sausage, and vegetables on a separate platter.