Pre-heat the oven to 190 degrees. Arrange the chopped onion, celery and carrot in the base of a large, ovenproof casserole dish. Sit the chicken on top with the tied leek to one side of it.
Pour the chicken stock over and tuck the bay leaf and some sprigs of thyme in and around the chicken and vegetables. Place over a moderate heat and bring to a simmer, then cover the chicken with aluminium foil and transfer to the oven.
Roast for 30 minutes then remove the foil and baste the chicken with the surrounding stock. Replace the foil and roast for a further 30 minutes until the chicken is cooked; pierce a thigh and if the juice runs clear, the chicken is cooked. Remove and allow to rest for 5 minutes before serving.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.