Rinse the chicken carcasses and then put them into a 5-quart stockpot. Pour on enough water to cover the bones generously and add the remaining ingredients. Bring to the boil, skimming away any fat and impurities that rise to the surface, then lower the heat immediately. Cover the pot and simmer very gently for 2 hours, skimming from time to time.

Carefully ladle the stock into a container, discarding the solids. (Don't be tempted to dump everything into a sieve: the less you disturb the solids at this stage, the clearer the final broth will be. I find the best method is to use a sieve to keep the solids submerged while you ladle out the liquid.)

Refrigerate for up to three days or freeze for later use.