Let thawed turkey come to room temperature for 45 minutes to 1 hour before roasting. This helps to promote even cooking.
Adjust oven racks so that the turkey will fit in the center of the oven (you do not want the top of the turkey to be too near the top of the oven). Preheat oven to 325° F.
Make herb butter: In a medium bowl, stir 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.
Place turkey on a rimmed baking sheet or in a disposable roasting pan (to catch any juices) and remove it from the packaging. Remove the neck and bag of giblets from the cavity. (Check both the big cavity and the smaller neck cavity to make sure you remove everything. The bag of giblets is often in the neck cavity.)
Pat turkey dry with paper towels, including inside the cavity. Generously season the cavity with salt and pepper. Use your fingers to gently loosen the skin from the breasts (be careful not to tear it) so that you will be able to rub some of the herb butter underneath. Tuck the wings underneath the turkey.
Place the quartered onion, ½ head of garlic, quartered lemon and a few sprigs of fresh rosemary and thyme in the cavity. If your turkey is on the smaller side, you may not fit all of the onion and lemon, just put what fits without overstuffing the cavity.
Put a generous amount of the herb butter under the skin on each of the turkey breasts. Rub your fingers over the skin of the breasts to spread the butter around. Then rub the rest of the herb butter all over the outside of the turkey.
Use kitchen twine to tie the legs of the turkey together, close to the body of the turkey.
Place turkey on a rack set inside a large roasting pan, breast side up. (See the note below for instructions on using a roasting pan without a rack, such as a disposable roasting pan.)
Bake turkey at 325° F for 13-15 minutes per pound, or until the internal temperature of the breast and thigh reaches 165°F. Begin checking on the turkey an hour before you expect it to be done; you don't want to overcook it or it will dry out. If the skin is browning too much as the turkey cooks, cover the top of the turkey loosely with foil.The turkey is done when an instant read thermometer inserted into the thickest part of the breast and thigh not touching bone registers 165°F. Let the turkey rest for 30-45 minutes before carving and serving. This allows the juices to redistribute throughout the meat for a juicy turkey. You can cover it with foil to keep it warm while it rests. (During this time I like to bake our side dishes in the oven and make the turkey gravy.) Find instructions for carving the turkey in the post above.
Save the juices from the pan to make turkey gravy.