Heat oven to 180°C. In a small bowl, mash the butter with one-third of the thyme leaves and the juice and zest of half a lemon. Mix everything together with some seasoning and set aside.
Place the chicken in a roasting pan, use your hand to loosen the skin away from the breast. Push most of the butter mix into the gap. Rub the remaining mix all over the outside of the chicken, then stuff the cavity with the lemon halves and remaining thyme.
Roast for 1½ hours, basting with the buttery juices after 30 minutes. When the chicken is crispy-skinned and the juices run clear, remove from the oven. Lift the chicken up, letting any juices run back into the tray. Transfer the chicken to a serving platter to rest for at least 10 minutes before serving.