Place the bones in a large saucepan or stockpot and add the water, or enough to cover the bones. Bring to the boil, skimming off the scum as it rises to the surface.
Tie together the thyme, parsley and bay leaf and add to the pan with the onions, celery, carrots and peppercorns. Cover and simmer gently for 2–3 hours.
Strain the stock through a sieve into a bowl, discarding the bones and vegetables. Leave to cool, then chill. Skim any fat from the surface and discard before using the stock.