Using a pestle and mortar or a mini processor, mash or blitz the garlic, lemon zest and thyme leaves to a coarse paste, stirring in the olive oil to loosen.
With a sharp knife make slashes over the flesh of the chicken and rub the mixture in with your hands, pushing it into the slashes all over the chicken.
Roast for 45 mins until golden and the juices run clear from the chicken when pierced with a knife.