Preheat your oven to 325°F (165°C) and place a rack in the lower third of the oven.

In a medium bowl, mix the softened butter, minced garlic, chopped rosemary, thyme, sage, salt, and black pepper until well combined to form a herb paste.

Pat the turkey completely dry with paper towels inside and out, then place it breast-side up on a roasting rack inside a large roasting pan.

Gently loosen the skin over the breast and thighs using your fingers, and rub about one-third of the herb butter mixture directly onto the meat under the skin.

Rub the remaining herb butter evenly all over the outside of the turkey skin and drizzle with olive oil.

Stuff the cavity of the turkey with the halved lemon, garlic head, rosemary sprigs, and quartered onion.

Roast in the preheated oven for approximately 3 to 3.5 hours, or until an instant-read thermometer inserted into the thickest part of the thigh reaches 165°F (74°C).

Remove the turkey from the oven, tent loosely with aluminum foil, and let it rest for 20 to 30 minutes before carving and serving.