You can, if you wish, barbecue a whole chicken, though the bird needs to be spatchcocked first -- doing so flattens it so it will cook evenly on the barbecue.
To spatchcock a chicken, using a good pair of kitchen scissors or poultry shears, place the chicken, breast-side down, on a board and, starting at one end, cut the
chicken all along the backbone so that you can open it up. Turn it around to place it breast-side up and press the chicken down to flatten it. Some people put skewers through the bird for stability; this is handy, but it is not completely necessary.
Next, slash each chicken leg two or three times with a sharp knife. Season with some salt and freshly ground black pepper, then sprinkle with the chopped rosemary or thyme, whichever you're using. Add a drizzle of extra-virgin olive oil. Now you can cook the chicken either in the oven or on the barbecue.
If you're cooking the chicken in the oven, transfer the spatchcocked bird to a roasting tin. Turn the skin side upwards and tuck the whole garlic cloves underneath. Roast in an oven preheated to 180 C, 350 F, Gas 4. Depending on the size of the bird, it should take about 50-60 minutes to cook. When it is fully cooked, the legs will feel loose and there will be no trace of pink in the juices when you pierce the chicken with a skewer.
If you're cooking on the barbecue, place the chicken, breast side down, on the direct heat of a hot barbecue, and cook for five minutes, then turn it over and cook for five minutes on the other side. Then move the chicken away from the direct heat, put the garlic cloves underneath, and place the lid on the barbecue. The bird will take about 45-60 minutes, depending on the heat of the barbecue, to fully cook. Turn the chicken over from time to time to make sure that it doesn't burn.