Put the chicken bones, onion, carrots, celery and 14 cups of water in a large heavy pan. Bring to a slow boil. Skim the foam off the top as needed. Add the bouquet garni and peppercorns. Reduce heat and simmer 3 hours. Occasionally skim the foam off the top and give it a little stir.

Discard the bones and veggies. Ladle the stock in little batches into a fine sieve over a bowl. Press the solids down with the back of the ladle to get all the liquid out.

Set the stock aside to cool. Refrigerate until cold and spoon off any fat that has risen to the top. Use within 48 hours or freeze for up to 6 months.