In a very large soup pot (I used a 10 quart stock pot), add the chickens. On top add the onions, carrots, parsnips, celery, fennel, garlic, parsley, thyme, rosemary, dill, bay leaves, peppercorns and salt. Cover with water so that everything is just submerged. You may need more or less than 5 quarts of water.
Place the pot on the stove and bring to a boil. Lower the heat so that the stock simmers and simmer for 5-6 hours, skimming the fat that accumulates at the top periodically.
Remove from heat and let stand for about an hour, or until it's comfortable to handle. Pour the stock through a colander into a large bowl. Then, with a cheesecloth fitted over a mesh strainer, strain the chicken stock again, into another large bowl.
Set aside to cool completely, then portion out into freezer safe containers. You can refrigerate the chicken stock for about a week or freeze it for up to three months.