Sit the whole chicken in a large stockpot with the halved onion, carrot, celery, bay leaves, peppercorns and parsley stalks. Cover with the cold water.
Bring slowly up to temperature over a medium heat. As foam rises to the surface in the first 10 minutes, skim it off with a spoon and discard.
As soon as the first bubbles appear, turn the heat right down. **The broth must never boil** — you want the surface barely moving. Simmer very gently, uncovered, for 1 1/2 hours.
Lift the chicken out onto a board and leave until cool enough to handle. Strain the broth through a sieve into a clean pot and discard all the aromatics.
Pull the meat off the chicken, discarding skin and bones, and shred it roughly. You’ll get about 500 g; use half in the soup and keep the rest for sandwiches.
Skim any fat from the surface of the strained broth, or refrigerate it and lift the set fat off if you have time.
Bring the broth back to a gentle simmer. Add the sliced carrot, celery, leek and garlic and cook for 15 minutes until just tender.
Return the shredded chicken to warm through for 2 minutes. Season generously — a long broth needs more salt than you expect.
Stir through the parsley and ladle over cooked noodles or rice in the bowls.