Slice the lemon halfway through lengthways. Insert a sprig of thyme into the slit and place the lemon inside the cavity of the chicken.
Make the flavoured oil. Strip the leaves from the remaining thyme stalks and place the leaves in a bowl.
Press the garlic cloves with the heel of your hand to lightly crush. Remove the skins and add the garlic cloves to the bowl.
Pour the olive oil into the bowl.
Using a pestle and mortar, a mini food processor or a hand blender, blend together the thyme leaves, garlic and olive oil. It should be quite liquid.
Spoon the thyme, garlic and oil mixture all over the chicken, working it into all the nooks and crannies.
Place the chicken in a roasting tray and pour over the remaining flavoured oil. Season the chicken generously with sea salt and black pepper.
Cover the chicken loosely with foil and roast in the oven for 30 minutes. After 30 minutes, remove the foil and roast for another 20-30 minutes and reduce the temperature to 160C-180C/320F-350F/Gas 3-4. It's fully cooked when the juices run clear when the bird is pierced in the thigh with a skewer.
Turn off the oven and let the bird rest in the residual heat for at least ten minutes. Remove from the oven, then carve.
Stir the pan juices over the hob to heat through.
Serve the carved chicken on a large platter and pour the pan juices over the top.