Pick all the good meat off the poultry carcass and place the bones into a large stock pot. Break the bones apart as much as you can to make it fit compactly. Add water to just cover, then add an onion cut in half, carrots, celery (including greens), any vegetable scraps you have on hand, plus fresh parsley, garlic cloves (optional for a stronger flavor), 10 whole peppercorns and 2 or 3 bay leafs. Bring it to a boil, then lower heat and simmer, uncovered, for 4 hours.
Remove from heat and let cool for 15 or 20 minutes. Remove and discard all the large solids, and pour the stock through a mesh strainer to remove the small stuff. Refrigerate overnight. The next day skim the fat off the surface and freeze.