Step 1Trim the excess fat off the beef and pork pieces. Tie each piece so that it will hold its shape during cooking.
Step 2Truss the chicken. To each piece of meat and to the chicken, tie a string long enough to fasten to the handle of the large stockpot, so that the meats may be removed easily for testing. Set aside the pork and chicken.
Step 3Place the beef in a stockpot large enough to accommodate the meats and vegetables. Add the soup vegetables -- 3 carrots, 3 onions, the parsnips, celery stalks and leeks.
Step 4Tie the 6 parsley sprigs, the bay leaf, thyme, garlic and peppercorns in cheesecloth and add to the pot. Add the beef and chicken broth and enough water to cover the meat and vegetables by 6 inches. More liquid may be added later if necessary. Set the kettle over moderate heat, bring to a simmer and skim. Partially cover the kettle and simmer slowly for 1 hour, skimming occasionally.
Step 5Add the pork and chicken. Bring the mixture quickly back to a simmer. Skim. Simmer 1 1/2 hours, skimming from time to time. Pull up the chicken and check to see if it is done. If it is done, remove it to a bowl and keep moist with several ladlefuls of stock.
Step 6To prepare the vegetable garnish, peel the turnips and quarter the turnips and the remaining 3 to 4 carrots lengthwise. Leave small carrots whole. Tie the carrot and turnip quarters and the remaining 3 to 4 onions and 3 to 4 leeks in one or several bundles of cheesecloth so they may be removed easily from the pot.
Step 7Add the vegetable garnish and bring the pot quickly back to a simmer. Taste the stock for seasoning and salt lightly if necessary. Simmer 1 to 2 hours, adding the sausage half an hour before the end. The meats and chicken are done when they are tender if pierced with a sharp-pronged fork or skewer. If any piece is tender before the others are done, remove to a bowl and keep moist with several ladlefuls of cooking stock. Return to the pot to reheat before serving.
Step 8To serve, remove the meats and vegetable garnish from the stockpot. Discard the trussing strings. Arrange the vegetables on a large, hot platter and moisten them with a ladleful of cooking stock. Decorate with parsley. Either place the meats in a large casserole for carving at the table or carve in the kitchen and arrange on a platter. Strain, degrease and season enough cooking stock to fill a large serving bowl, and pass it along with the Dijon mustard.