Preheat the oven to 220°C/425°F/Gas 7. Put the chopped-up chicken and the onions in an oven dish and roast for 1 hour, stirring from time to time. After 40 minutes add the carrots, celery and garlic. Once the bones have gone a dark brown colour – but not burnt – tip them and the vegetables into a large pot (at least 8 litres or 14 pints). Add the rest of the ingredients and bring to the boil. Pour an extra 600ml (1 pint) boiling water into the roasting dish and leave to soak for 30 minutes, then scrape the dish and add the contents (water and all) to the pot as well. Skim any scum from the surface of the stock and simmer for 3-5 hours. Strain well and it’s ready to use.