Place the quartered onion in a large, heavy-bottomed stockpot along with half of the leeks, celery, and carrots. Arrange the beef shanks and tied rump roast over the bed of vegetables. Gather the parsley, thyme, and bay leaves into a bundle using a small square of damp cheesecloth and tie it securely with kitchen twine. Drop the herb sachet into the pot along with the peppercorns and kosher salt. Add two marrow bones if using, then pour in the 8 quarts of water to fully submerge the ingredients.

Bring the mixture to a steady boil over high heat. Once boiling, reduce the heat to low and partially cover the pot. Allow the broth to simmer gently for about 2 ½ hours, skimming off any foam or impurities that rise to the surface. The meat should be fork-tender and richly infused with the flavors of the aromatics.

Carefully remove the beef shanks and rump roast and transfer them to a large bowl. Cover to keep warm. Strain the broth through a fine sieve, discarding the spent vegetables and herbs. Return the clear broth to the pot and bring it to a boil over high heat. Continue boiling until reduced to about 10 cups, skimming off excess fat as needed—this will take roughly 45 minutes.

Add the remaining carrots, leeks, and celery to the clarified broth, followed by the parsnips, turnips, and rutabagas. Cover and simmer over low heat for 30 minutes, or until the vegetables begin to soften. Add the remaining marrow bones and the scrubbed potatoes, cover again, and continue cooking for another 40 minutes until the potatoes are tender all the way through.

Remove the string from the rump roast and slice it against the grain into 6 to 8 portions. Cut the shank meat into hearty 2-inch chunks. Return the meats to the pot and let them warm through in the fragrant broth for a few minutes. Season the mixture generously with salt and freshly ground black pepper to taste.

To serve, ladle the hot broth into wide, shallow bowls. Add slices of meat, marrow bones, and tender vegetables to each bowl. Serve immediately, offering horseradish, coarse mustard, and sour cream on the side for guests to mix as they please.