Preheat the oven to 163°C.
Drain any juices from the turkey and place it on a flat rack in a large, shallow roasting pan. Pat the turkey dry with paper towels.
Working from the large cavity end, gently run your fingers between the skin and the meat to loosen the skin, taking care not to tear the skin. (Be careful not to do this with rings on your fingers!) Place the softened butter under the skin and gently massage the skin to spread the butter evenly over the whole breast.
Sprinkle the cavity generously with salt and pepper. Place half of the thyme sprigs in the cavity along with the onion and garlic. Tie the legs together with kitchen string and tie 1 length of string around the bird and the wings to keep the wings close to the body.
Brush the turkey with olive oil, sprinkle with 2 teaspoons of salt and 1 teaspoon of pepper. Remove 1 tablespoon of thyme leaves from the stems, chop them, and sprinkle on the turkey. Roast the turkey for 2 1/2 to 3 hours, until an instant-read thermometer placed in the center of the breast registers 70°C. If you want the skin to be crisp, don't baste the turkey at all! About halfway through, when the breast is golden brown, cover the breast loosely with aluminum foil to prevent the skin from burning.
Remove from the oven, cover tightly with aluminum foil and allow to rest for 15 to 20 minutes. Carve and serve with the pan juices.