Preheat the oven to 325 degrees. Drain any juices from the turkey cavity and place the turkey on a rack in a large, shallow roasting pan, breast side up. Pat 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 it. (Don't do this with rings on your fingers!) Place the softened truffle 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 the onion, garlic, and half of the thyme sprigs in the cavity. Tie the legs together with kitchen string and tie one length of string around the bird and the wings to keep the wings close to the body.
Brush the turkey all over with olive oil and sprinkle with 2 teaspoons of salt and 1 teaspoon of pepper. Remove 1 tablespoon of leaves from the remaining thyme sprigs, chop them, and sprinkle on the turkey. Roast the turkey for 2 1/2 to 3 hours, until a meat thermometer placed in the center of the breast registers 160 degrees. 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 the turkey from the oven, cover tightly with aluminum foil, and allow to rest in the pan for 15 to 20 minutes. Carve and serve with the pan juices.
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