Position a rack in the centre of the oven and preheat the oven to 220°C. Place the chicken on a wire rack set on a large, heavy, rimmed baking tray. Using your fingers, separate the skin from the breast meat. Place 4 sage leaves and butter under the skin of each breast. Tie the legs together with kitchen twine. Rub 2 tbs of the oil all over the chicken and season it generously with salt and freshly ground black pepper. Roast for 1 hour 10 mins or until a meat thermometer reads 75°C when inserted into the thickest part of the breast, the juices run clear when pierced, and the skin is crisp and golden. Transfer the chicken to a platter and rest for 10 mins before serving.
During the last 30 mins of roasting the chicken, prepare the vegetables. In a large bowl, toss the sweet potato, parsnips, garlic, thyme and remaining 12 sage leaves with the remaining 1 tbs oil to coat. Season with salt and pepper. Transfer the vegetable mixture to a 30cm x 25cm x 5cm baking dish. Roast the vegetables, turning occasionally, for 30-35 mins or until the vegetables are tender and deeply caramelised all over.
Pour the drippings from the roasted chicken into a small liquid measuring cup. Spoon off the fat from the drippings, reserving 1 tbs of the fat separately from the pan drippings. In a small, heavy saucepan, heat the reserved fat over medium-high heat. Whisk in the flour. Continue whisking for about 1 min to cook the flour. Whisk in the reserved pan drippings, stock and any juices that have accumulated on the platter. Simmer, whisking constantly, for about 2 mins or until the gravy thickens slightly. Season with salt and pepper.
Serve the roasted chicken and vegetables with the gravy and roasted garlic squeezed from their skins.