Position a rack in the upper third of the oven and preheat to 425 degrees . Place the chicken in a large roasting pan, rub with 1/2 tablespoon olive oil and season with salt and pepper. Roast for 10 minutes.
Meanwhile, in a large bowl, toss the potatoes, carrots and parsnips with the remaining 2 tablespoons olive oil and season with salt and pepper.
Place the vegetables around the chicken and scatter 8 thyme sprigs on top. Roast until the vegetables are tender and the chicken juices run clear when a leg is pricked, about 40 minutes. Pick the leaves from the remaining 2 thyme sprigs and sprinkle on top.