Remove the chicken from the refrigerator and let stand at room temperature for 1½ to 2 hours, nor until it comes to room temperature.
Preheat the oven to 475°F.
Remove the neck and innards if they are still in the cavity of the chicken. Use these bits for a giblet pan gravy (see separate recipe).
Using a paring knife, cut out the wishbone from the chicken. (this will make it easier to carve the chicken.). Save this for stock (see separate recipe).
Generously season the cavity of the chicken with salt and pepper, add 3 of the garlic cloves and 5 sprigs of thyme, and massage the inside of the bird to infuse it with the flavors. Truss the chicken (see page 23 of Ad Hoc at Home for instructions on how to truss the chicken).
Cut off the dark green leaves from the top of the leeks. Trim off and discard any darkened outer layers. Trim the root ends, cutting around them on a 45-degree angle. Rinse the leeks well under warm water.
Cut off both ends of the rutabagas. Stand the rutabagas on end and cut away the skin, working from top to bottom and removing any tough outer layers. Cut in ¾-inch wedges. Repeat with turnips, cutting the wedges to match the size of the rutabagas.
Combine all the vegetables and the remaining garlic cloves and thyme spring into a large bowl. Toss with ¼ cup of the oil and season with salt and pepper. Spread the vegetables in a large cast-iron skillet or a roasting pan.
Rub the reaming oil all over the chicken. Season generously with salt and pepper.
Make a nest in the center of the vegetables and nestle the chicken in it. Cut the butter into 4 or 5 pieces and place over the chicken breast.
Put the chicken in the oven and roast for 25 minutes.
Reduce the heat to 400°F and roast for an additional 45 minutes, or until the temperature registers 160°F in the meatiest portions of the bird - the thighs, and under the breast where the thigh meets the breast, and the juices run clear. If necessary, return the bird to the oven for more roasting, check it every 5 minutes.
Transfer the chicken to a carving board and let rest of 20 minutes.
Just before serving, set the pan of vegetables over medium heat and reheat the vegetables, turning them and glazing the pan with the pan juices. If making a pan gravy, transfer the vegetables to a serving dish and following the instructions for deglazing the pan for tasty gravy.
Cut the chicken into serving pieces, arrange over the vegetables, and serve.