1. Place the oxtails, rump roast, beef stock, and 8 cups of the chicken stock in a 12-quart or larger stockpot. Set the pot over medium heat.
2. While the beef is coming to a simmer, divide the carrots into 2 bunches and tie each bunch in two places with kitchen twine. Add 1 bunch of carrots to the pot and set the other bunch aside on a tray. Stud each of the onions with 2 cloves. Add half of the on- ions to the pot and the other half to the tray with the carots. Add 1 of the celery stalks to the pot. Make a bouquet garni by tying 1 of the remaining celery stalks with half of the parsley sprigs and half of the thyme sprigs. Repeat with the remaining celery stalk, parsley, and thyme sprigs. Add 1 bou- quet garni to the pot and the other to the tray. Add half of the bay leaves to the pot and the other to the tray. Make 2 cheese cloth pouches, each holding half of the peppercorns and 1/2 head of garlic. Add 1 bundle to the pot and the other to the tray.
3. Once the broth comes to a simmer, reduce the heat to low so that it continues to cook at a gentle simmer. The heat should be low enough that the bubbles burst in one spot. As the pot simmers, foam will rise to the top. Have a large ladle and container near the stove. Remove the foam by skimming the surface of the broth and discarding it into the container as needed. The longer the broth simmers, the lighter in color the foam will become. Allow the beef to cook gently for 1 hour, checking the heat, never allowing the broth to come to a rolling boil, and skimming as necessary. At the end of cooking you will have a clear and flavorful broth.
4. Add the pork butt to the pot and raise the heat to medium. Once the broth returns to a simmer, reduce the heat to low and continue to cook for 1 hour longer, skimming the broth as necessary.
5. Using long tongs, carefully remove the vegetables, the herb bundle, and the cheesecloth bundle from the pot and discard. Add the chicken, the sausages, the remaining 4 cups chicken stock, and the ingredients reserved on the tray. Raise the heat to medium and add the salt. Once the broth returns to a simmer, reduce the heat to low and simmer gently for 20 minutes, skimming as necessary. Add the leeks and turnips and continue to simmer for 40 minutes longer. Turn off the heat.
6. Set a large carving board and a large serving platter next to the pot. Using long tongs, very carefully remove the chicken, pork butt, rump roast, and oxtails and set aside on the carving board. Remove the strings. Remove the onions from the pot and discard. Carefully remove the remaining vegetables and sausages and arrange them in neat piles on the serving platter. Remove the strings. If desired, halve the carrots.
7. Set a chinois or other fine-mesh sieve over a large bowl or pot and strain the broth. You will have a clear, flavorful broth. If you would like to refine your broth further, you may strain the broth again through several layers of cheesecloth. Ladle some of the broth over the vegetables. Slice the rump roast into 6 to 8 slices and arrange on the platter. Slice the pork butt into 6 to 8 slices and arrange on the platter. Carve the chicken and place on the platter as well. Remove the oxtail meat from the bones and arrange on the platter. Moisten the meats with some of the broth.
8. Serve the guests individual small bowls of hot broth and Caper Parsley Aļoli to accompany their choice of meats. Serve the pot-au-feu family style, passing the peppermill and coarse sea salt.