Preheat your oven to 325°F (163°C).
Remove the giblets from the turkey's cavity and pat it dry with paper towels. Season the inside of the turkey with kosher salt and black pepper. Stuff it with the quartered onion, lemon, and fresh herbs (rosemary, thyme, and sage).
In a mixing bowl, combine unsalted butter, minced garlic, finely chopped rosemary and thyme, and salt and black pepper. Mix well.
Carefully separate the skin from the turkey breast by gently sliding your fingers or spoon between the skin and the meat. Spread a generous amount of the herb butter under the skin of the turkey breast. This will help keep the breast meat moist and flavorful. Then, spread the remaining herb butter all over the entire outside of the turkey. Season the outside of the turkey with additional salt and black pepper to taste.
In the bottom of a roasting pan, place the quartered onion, chopped celery, and chopped carrots. These will serve as a flavorful bed for the turkey.
Place the seasoned turkey on top of the bed of vegetables in the roasting pan. Cover the turkey loosely with aluminum foil and transfer it to the preheated oven. Roast until the internal temperature of the turkey reaches 160°F in the thickest part of the thigh; it will continue to rise to 165°F as it rests outside the oven. The exact cooking time depends on the size of your turkey; for reference, mine took 3 hours. During the final 30 minutes, remove the foil to allow the turkey to achieve that desirable golden brown color.
Once the turkey is done, remove it from the oven, tent it with foil, and allow it to rest for at least 30 minutes. This allows the juices to redistribute, resulting in a moist and flavorful turkey. Then, carve the turkey and serve it with your favorite Thanksgiving side dishes.