The day before cooking the turkey, remove the giblets from the turkey cavity and discard. Wash the turkey thoroughly and pat the skin dry with paper towels. Place on a wire rack over a baking sheet and refrigerate, uncovered, for 12 hours. This process makes for a crispy skin.
Bring the turkey to room temperature before cooking, about an hour. Preheat the oven to 450 degrees F.
Put the turkey in a roasting pan with a rack. Season the cavity with 2 tablespoons salt and 1 tablespoon black pepper. Put the thyme sprigs, parsley, sage, apple and onion in the cavity of the bird. Gently loosen the skin over the breast with your fingers. Rub the breast meat under the skin on both sides with 2 tablespoons of butter.
In a small bowl, mix together the rosemary, chopped thyme and Herbes de Provence. Brush the oil over the entire turkey and rub with the mixed herbs and the remaining 1 tablespoon salt and 1 tablespoon black pepper. Pour 2 cups of chicken stock in the roasting pan.
Cook the turkey breast-side up for 30 minutes, then reduce the heat to 325 degrees F. Roast until an instant-read thermometer inserted in the thickest part of the breast reads 165 degrees F, 3 to 3 1/2 hours, basting every 30 minutes with drippings from the pan. If the juices begin to dry out, add 1/2 cup of the chicken stock each time you baste. Let the turkey rest for 30 minutes before carving.
Strain the drippings from the roasting pan through a sieve into a saucepan and set over medium-high heat. Add enough chicken stock to the drippings to make 3 cups of liquid and bring to a boil. Mix the cornstarch with 2 tablespoons cold water in a small bowl. Pour the cornstarch mixture into the boiling gravy, reduce the heat to low and cook for another minute. Keep warm until ready to use. Just before serving, whisk in the remaining 2 tablespoons butter.