Arrange a rack at center position and preheat the oven to 325 degrees.
Season the cavity of the turkey generously with salt and pepper. Combine the leeks and apples in a medium-sized bowl, and place 1 cup of this mixture in the cavity along with the sage and thyme sprigs.
In a medium-sized bowl, mix together the butter, dried sage, dried thyme, 1 ½ teaspoons salt, and 1 teaspoon pepper until well blended. Reserve 2 tablespoons of this butter for the sauce and set aside. Use 4 tablespoons of the remaining herb butter and rub it over the surface of the turkey. Truss the turkey: Use a long piece of kitchen twine and tie the legs together, slightly overlapping, then bring the string around the sides of the bird, pulling the wings toward the body, and tie the twine to secure.
Place the turkey on a rack set in a large flameproof roasting pan. Spread the remaining leeks and apples on the bottom of the pan. Combine the cider and white wine and reserve 1 ½ cups of this mixture for the sauce. Pour 1/3 cup of the remaining cider mixture over the turkey.
Roast the turkey until golden brown and a thermometer inserted into the thickest part of the thigh registers 180 degrees, basting every 30 minutes with 1/3 cup of the cider mixture and 1 tablespoon of the herb butter, for about 3 hours and 10 minutes or longer. Watch carefully and if the turkey begins to brown too much, cover it loosely with foil. When done, move the turkey to a platter and tent it loosely with foil. Let rest for 30 minutes while you prepare the sauce.
Drain the liquids from the roasting pan into a bowl, pressing down hard on the roasted leeks and apples to release all the juices; discard the solids. Skim off and discard any fat from the drippings and return the drained juices to the pan. Add the reserved cider mixture and any leftover cider mixture from basting to the pan and place it over 2 burners set on medium-high heat. Using a whisk, scrape up any brown bits on the bottom of the pan into the liquids. Cook, whisking often, until the liquids have reduced by about 1/3, about 5 minutes.
Combine the reserved 2 tablespoons herb butter and any butter left over from basting with 2 tablespoons of flour and blend with a fork to make a paste(if you want the sauce to be thicker, increase the flour to 3 tablespoons). Whisk the butter-flour mixture into the pan a little at a time until the sauce thickens and coats the back of a spoon, 5-6 minutes. Season the sauce with salt if needed and transfer it to a serving bowl.
To serve, remove and discard the twine and the ingredients from the cavity. Garnish the turkey with bouquets of sage and thyme sprigs. Pass the sauce on the side.