About 1 hour before roasting, remove your fully thawed turkey from the refrigerator and let it rest at room temperature. This takes the chill off which helps the butter mixture stick properly and the turkey cook more evenly.
Adjust the oven rack so that your turkey will sit in the center of your oven. If your roasting your turkey in a convection oven, position the rack in the lower position so that the top of your turkey is not too close to the convection fan.
Preheat the oven to 325°F (165°C).
Make the herb butter by combining ¾ cup softened unsalted butter, 2 tablespoons of olive oil, 4 cloves minced garlic, 2-3 teaspoons salt, 1 teaspoon pepper, 1 teaspoon paprika, and 2 teaspoons poultry seasoning in a small bowl. Mix until smooth and set aside.
Remove the giblets and neck from the inside of the turkey (save them to make gravy or turkey stock if you'd like, or discard), then pat the turkey completely dry with paper towels.
Gently loosen the skin over the breast using your fingers to create space for the butter. Spread about ⅓ of the herb butter mixture under the loosened skin, then rub the remaining butter all over the outside of the turkey.
Place the onion, apple, and fresh herbs inside the turkey cavity. Tie the legs with kitchen twine (optional) and tuck the wing tips underneath the turkey to prevent them from burning.
Scatter the chopped carrots, celery, and any remaining onion in the bottom of a large roasting pan. Add 2 cups of turkey or chicken stock.
Place the turkey on the roasting rack positioned inside the roasting pan.
Roast for about 13 to 15 minutes per pound. The turkey is ready when a meat thermometer registers 160°F in the thickest part of the breast (or 165°F to 170°F in the thigh). Remove from the oven. Check the turkey about halfway through roasting. Once the skin is a deep golden brown, loosely tent the top with foil to protect the breast meat from overcooking. Or, start covered and remove the foil during the final hour to allow the skin to brown.
Transfer the turkey to a large cutting board placed inside a large rimmed baking sheet to catch any excess drippings. Tent loosely with foil, and let it rest for at least 30 minutes before carving.
Carve and serve the turkey. Remember to reserve any juice and drippings to make homemade turkey gravy.