1. In a 6- or 8-quart saucepan or soup kettle, combine the beef and chicken parts and cover them with the water. Add the salt. Bring to a boil over high heat, adding more water, if necessary, to cover. Skim off surface scum as it rises.
2. Meanwhile, in a heavy 12-inch skillet, heat the butter. When the foam subsides, add the chopped onions, parsnip, carrots, celery, leek and parsley.
3. Over high heat, toss the vegetables in the hot butter for 4 or 5 minutes, or until they are lightly browned. Scrape them into the soup kettle and bring the liquid to a boil again, skim off the surface scum and add the bay leaf, peppercorns and allspice.
4. Turn the heat to its lowest point, partially cover the pot and simmer slowly for about 2 hours, or until the beef shows no resistance when pierced with the point of a small sharp knife.
5. Remove the beef to a heated serving platter. Then skim the surface fat from the stock and strain the stock through a large sieve, pressing down hard on the vegetables before discarding them. Taste for seasoning.
6. The stock may be served as a soup before the beef or on another occasion with dumplings that have been cooked in it.
