To make the turkey stock, put the carcass pieces into a large pan with the onion, carrot, celery and herbs. Pour in 2 litres of cold water and gently bring to the boil. Skim fat or scum from the surface then simmer gently for 1 hour. Strain through a fine sieve. Once cool enough to handle, remove as much meat as possible from the carcass, disposing of the bones and vegetables. This can be done up to 3 days in advance or frozen with any left-over meat.