In a large pot, combine the onion and half each of the leeks, celery and carrots. Set the beef shanks and rump roast on top of the vegetables. Wrap the parsley, thyme and bay leaves in a piece of cheesecloth and tie into a bundle. Add the bundle to the pot along with the peppercorns and 1 tablespoon of kosher salt. Add the water. Bring to a boil over high heat, then reduce the heat to low, cover partially and simmer, skimming occasionally, until bottom round is very tender, about 2 1/2 hours.
Transfer the shanks and roast to a large bowl and cover. Strain the broth and return it to the pot. Boil over high heat until reduced to 10 cups, about 45 minutes; skim off the fat.
Add the remaining leeks, celery and carrots to the broth along with the parsnips, turnips and rutabagas. Cover and simmer over low heat until the vegetables are just tender, 30 minutes. Add the potatoes. Cover and simmer until the potatoes are tender, 40 minutes.
Untie the bottom round roast and cut it across the grain into 6 to 8 slices. Cut the shank meat into 2-inch chunks; add the meats to the pot and simmer until heated through. Season with salt and pepper. Ladle the broth into shallow bowls. Add the meats and vegetables and serve, passing horseradish, mustard and sour cream at the table.