Preheat the oven to gas 6, 200°C, fan 180°C. Season the chicken and stuff the cavity with 2 of the lemon halves and the fresh thyme.
Put the chicken into a large roasting tray along with the garlic halves. Rub the butter over the breast of the chicken and cook in the oven for 1 1/2 hours, basting occasionally until completely cooked and the juices run clear when a skewer is inserted.
Remove the chicken from the roasting tray and mix the garlic into the juices. Squeeze in the juice from the remaining lemon and stir well. Serve the juices with the chicken.
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