Place the chicken, carrots, celery, potatoes and thyme into a large, deep pan and pour in 3 litres of water or enough to cover the chicken.
Simmer on a medium heat for 1½ hours or until the chicken is cooked through.
Remove the chicken from the pan and strain the broth. Save the veg for later. Put the broth back on a high heat and allow to reduce for 15 minutes until there’s about 2 litres left.
Meanwhile, tear the cooked chicken into long chunks. Once the broth has reduced, throw the vegetables back in the pan with the peas, leeks and chicken. Simmer for a further 5 minutes, then remove the sprigs of thyme.
Serve in warm bowls sprinkled with chopped parsley and the celery leaves.
The chicken is the star ingredient so use the best you can get your hands on.