If your turkey is frozen, thaw it in the fridge, allowing roughly 24 hours for every 5 pounds of weight. Once thawed, remove the giblets and pat the entire bird dry, inside and out, with paper towels. Drying the skin thoroughly is the real secret to crispy, golden results later.
Rub olive oil all over the skin, then massage the softened butter underneath and over the surface for extra richness. Sprinkle the salt, pepper, thyme, and rosemary generously, making sure to season both the outside and the cavity. Stuff the lemon, onion, and garlic inside the turkey for flavor from within.
Tie the legs together loosely with kitchen twine so the turkey holds its shape while roasting evenly. Place the rack inside your roasting pan and set the turkey breast-side up on top of it. Pour the stock into the bottom of the pan to keep the oven environment moist throughout.
Preheat your oven to 350°F and place the turkey on the middle rack, uncovered at first for better browning. As a rule of thumb, roast for about 13 minutes per pound, so a 15-pound turkey needs roughly 3 hours 15 minutes. Tent loosely with foil halfway through if the skin starts browning too quickly.
Insert your instant-read thermometer into the thickest part of the thigh without touching the bone for an accurate reading. The turkey is done when it reads 165°F in the thigh and the breast meat reaches around 160°F. Pull it from the oven right at this point since it will continue cooking slightly while resting.
Let the turkey rest, loosely tented with foil, for at least 20 to 30 minutes before you even think about carving it. This resting period allows the juices to redistribute through the meat instead of spilling out onto your cutting board. Once rested, carve the turkey using a sharp knife and serve it warm.