Peel and halve the onions, then chop the leeks, celery and carrots into large chunks. Halve the garlic bulb around the equator. Put all the ingredients in a large pan with a pinch of sea salt, then cover with cold water. Place on the hob over a medium heat and bring just to the boil. Reduce the heat to a low simmer, skimming off any scum that rises to the top. Simmer gently for about 1 hour, or until the juices run clear when you pierce the meat. Remove the bird from the pan and bubble the poaching liquid for 40 minutes, or until reduced by about one-third (to add more flavour, you could strip the meat from the chicken and return the bones to the stock, too). Sieve the stock, then store in the fridge for up to 1 week.