Remove the turkey from its packaging, carefully removing the giblets and neck from both cavities. Pat the turkey very dry inside and out with paper towels. This step is crucial for crispy skin.

In a small bowl, combine the softened unsalted butter, finely chopped fresh rosemary, thyme, sage, garlic powder, onion powder, salt, and black pepper. Mix thoroughly until you have a cohesive herb butter paste.

Gently slide your hand under the skin over the turkey breasts and thighs to create pockets, being careful not to tear the skin. Evenly spread about half of the herb butter mixture under the skin. Rub the remaining herb butter mixture generously all over the exterior of the turkey.

Loosely stuff the main cavity of the turkey with the quartered yellow onion, roughly chopped celery, and quartered apple. Tie the turkey legs together with kitchen twine and tuck the wing tips underneath the body to prevent them from burning.

Place the prepared turkey on a V-rack set in a large roasting pan. Pour 4 cups of chicken or vegetable broth into the bottom of the roasting pan. This will add moisture and create pan drippings for gravy.

Preheat your oven to 375°F (190°C). Spread the 1-inch sourdough bread cubes evenly on a large baking sheet. Toast in the preheated oven for 10-15 minutes, or until they are lightly golden and dry. Remove from oven and set aside. Reduce oven temperature to 325°F (160°C) for turkey roasting.

In a large skillet or Dutch oven, melt 1/2 cup of unsalted butter over medium heat. Add the finely chopped yellow onions and celery. Cook, stirring occasionally, for 8-10 minutes until the vegetables are softened and translucent.

Add the peeled, cored, and diced Granny Smith apples to the skillet. Continue to cook for another 5 minutes, stirring occasionally, until the apples are slightly tender but still hold their shape.

Remove the skillet from heat. Transfer the cooked vegetable and apple mixture to a large mixing bowl. Add the toasted sourdough bread cubes, chopped fresh parsley, sage, thyme, salt, and black pepper. Toss gently to combine all the ingredients.

In a separate small bowl, whisk together the 2 lightly beaten eggs with 1/2 cup chicken or vegetable broth. Pour this mixture over the bread and vegetable mixture in the large bowl. Toss gently until all the stuffing ingredients are evenly moistened. If the stuffing appears too dry, add a little more broth, 1/4 cup at a time, until it reaches a moist but not soggy consistency.

Loosely fill the turkey's neck cavity with some of the prepared stuffing (do not overpack as it expands during cooking). For food safety, it is highly recommended to bake any remaining stuffing in a separate baking dish.

Cover the roasting pan containing the turkey loosely with aluminum foil, creating a tent to prevent the foil from touching the turkey skin.

Place the turkey in the preheated 325°F (160°C) oven. Roast for 2.5-3 hours, basting the turkey every hour with the pan juices to keep it moist.

After 2.5-3 hours, remove the foil from the roasting pan and increase the oven temperature to 375°F (190°C). Continue roasting for another 45-60 minutes, or until the turkey skin is golden brown and crispy. The most crucial indicator of doneness is temperature: a meat thermometer inserted into the thickest part of the thigh (without touching bone) should register 165°F (74°C). The stuffing inside the turkey should also reach 165°F (74°C).

For any separately baked stuffing, transfer it to a greased 9x13 inch baking dish. Bake alongside the turkey, uncovered, for the last 30-45 minutes of the turkey's cooking time, or until it's golden brown and heated through. Ensure it also reaches an internal temperature of 165°F (74°C).

Once the turkey reaches the target internal temperature of 165°F (74°C), carefully remove it from the oven. Transfer the turkey to a large cutting board and tent it loosely with aluminum foil. Let it rest for at least 20-30 minutes before carving. This critical step allows the juices to redistribute throughout the meat, resulting in a more tender and flavorful turkey.

Immediately after the resting period and before carving, remove any stuffing from the turkey cavity and combine it with the separately baked stuffing.

Carve the turkey and serve warm alongside the delicious Apple-Sourdough Bread Stuffing.