Let the thawed turkey sit at room temperature for 45 minutes to 1 hour before roasting to ensure even cooking.
Adjust oven racks so the turkey fits in the center of the oven, making sure the top isn’t too close to the heat source. Preheat the oven to 325°F.
Make the Herb Butter: In a medium bowl, mix together the softened butter, minced garlic, lemon zest (save the rest of the lemon for the cavity), chopped fresh thyme, chopped fresh rosemary, salt, and pepper.
Prepare the Turkey: Place the turkey on a rimmed baking sheet or in a disposable roasting pan to catch any juices. Remove it from its packaging and take out the neck and giblets bag from the cavity. (Check both the large body cavity and the smaller neck cavity.)
Pat the turkey dry with paper towels, including inside the cavity. Generously season the cavity with salt and pepper. Carefully loosen the skin over the breasts using your fingers, being cautious not to tear it. Tuck the wings underneath the turkey.
Stuff the cavity with the quartered onion, ½ head of garlic, quartered lemon, and a few sprigs of fresh rosemary and thyme. (If the turkey is small, don’t overstuff—just add what fits comfortably.)
Rub herb butter under the loosened skin of the breasts and spread it around by gently massaging over the skin. Then rub the remaining herb butter all over the outside of the turkey.
Tie the Turkey: Use kitchen twine to tie the legs together, securing them close to the body.
Roast the Turkey: Place the turkey breast-side up on a rack inside a large roasting pan. (If using a disposable roasting pan without a rack, see the note below for tips.)
Bake at 325°F for 13-15 minutes per pound, or until the internal temperature of the breast and thigh reaches 165°F. Start checking the turkey an hour before the estimated cook time to prevent overcooking. If the skin browns too quickly, loosely cover the top of the turkey with foil.
Check for Doneness: Insert an instant-read thermometer into the thickest part of the breast and thigh, avoiding bone. The turkey is fully cooked when it registers 165°F.
Rest the Turkey: Let the turkey rest for 30-45 minutes before carving. This allows the juices to redistribute, ensuring moist and tender meat. Keep it warm by covering it with foil while it rests. (Use this time to bake side dishes and make turkey gravy.)
Save the pan drippings to make turkey gravy.