Prepare the Turkey: Let the turkey rest at room temperature for 1 hour. Near the end of the resting time, transfer the oven rack to one level below center and preheat the oven to 350°F (175°C).
Clean the Turkey: Remove the neck and giblets from the turkey, as well as any pin feathers left behind. Pat the outside and inside of the turkey dry with paper towels. Tuck the wings under the turkey and season the cavity well with salt and pepper.
Prepare the Herb Butter: In a mixing bowl, combine the softened butter, lemon zest, thyme, and rosemary. Mix until well combined.
Stuff the Turkey: Carefully separate the skin from the breast of the turkey using the back end of a wooden spoon, being careful not to tear the skin. Transfer the turkey to a large roasting pan with a roasting rack. Using the spoon, stuff and spread about one-third of the herb butter under the skin, smoothing it out over the turkey.
Season the Turkey: Rub the remaining two-thirds of the butter all over the outside of the turkey (except for the underside). Season generously with salt and pepper.
Stuff the Cavity: Stuff the cavity of the turkey with the onion, garlic, and parsley. If desired, tie the legs together with kitchen twine for a better presentation.
Bake: Place the turkey in the preheated oven. Bake until the thickest part of the thigh registers 175°F (80°C) on a probe or instant-read thermometer and the breast registers 165°F (74°C), about 2 hours 20 minutes to 3 hours 30 minutes. Tent the top of the turkey with aluminum foil during baking as needed to prevent excessive browning.
Rest and Carve: Once done, let the turkey rest for 30 minutes (or up to 60 minutes) before carving.