Place the chicken onto a chopping board and cut in half down the backbone using a large and very sharp cooks’ knife. Flatten the chicken as much as you can buy pressing down with the palm of your hand.
Pour the olive oil into a mixing bowl large enough to take the chicken, add the squashed garlic cloves, the juice from the lemon and then the squeezed lemon shells. Put in the sprigs of thyme, a good seasoning of salt and black pepper, then turn the chicken over in the oil. (If you have the time, you can leave the chicken like this for an hour or so in the fridge.)
Place the chicken in a roasting tin, skin-side up. Roast for about 30 minutes or until the skin is golden and the bird cooked right through.