Preheat the Oven: Preheat the oven to 200C (400F).
Prepare the Chicken: Rinse the chicken and remove the giblets. Dry the chicken using kitchen paper.
Prepare the Lemon and Thyme: Cut the lemon in half and sandwich a sprig of the thyme in the middle. Put the lemon inside the chicken cavity.
Make the Olive Oil Mixture: Skin and crush the garlic cloves and add to a food processor with the leaves of the remaining thyme and the olive oil. Blend together.
Massage the Chicken: Massage the olive oil mixture into the chicken thoroughly, ensuring it is completely covered.
Season and Roast: Put the bird into a roasting tray, and top with any remaining oil. Season with sea salt and freshly ground black pepper. Cover the chicken loosely with foil and roast for 30 minutes.
Continue Roasting: Reduce the temperature to 170C (330F), remove the foil, and roast for a further 20 minutes, or until the juices run clear when the bird is pierced with a skewer.
Rest the Chicken: Turn off the oven, but allow the chicken to rest in the hot oven for ten minutes before removing it to carve.
Serve: Pour the juices from the roasting tray over the sliced chicken to serve, or set aside to make a gravy. The carcass can be used to make a chicken stock.