Preheat the oven to 200°C, fan 180°C, gas 6. Put the chicken, breast-side up, in a large roasting tin and rub all over with the cut side of the lemon half. Pop the lemon into the cavity of the chicken, along with the garlic bulb halves and parsley.
If the legs aren’t already tied, tie together with kitchen string. Smear the butter over the skin, then season and transfer to a roasting tin.
Roast for 1 hour 30 minutes to 1 hour 45 minutes, basting with the pan juices every so often, until the chicken is cooked through. To test it’s done, insert a skewer into the thickest part of the thigh; if the juices run clear, it’s cooked.
Remove from the oven, cover with foil and let rest for 15 minutes before carving.