Twelve to 24 hours before cooking the turkey, remove the giblets and dry the turkey inside and out with paper towels. Rub the Dry Brine on all sides of the turkey, completely seasoning the exterior as well as the interior cavity. Place the turkey in a pan inside the refrigerator overnight.
Arrange a rack in the center of the oven and preheat to 350 degrees F.
When ready to cook, remove the turkey from the refrigerator and pat dry with paper towels. Cut one of the onions into quarters. Place the onion quarters, herbs and one head of the garlic inside the turkey. Tie the legs together with a single piece of string and tuck the wings under the body.
Chop the remaining onion. Combine the chopped onion, carrots, celery and remaining garlic in a large roasting pan. Place the turkey on top of the vegetables in the roasting pan.
Brush the entire surface of the turkey with the melted butter and sprinkle lightly with salt. Pour any remaining melted butter over the vegetables in the base of the pan. Place the turkey in the oven to roast on the center rack with the legs towards the back wall of the oven.
After 45 minutes, remove the turkey from the oven and set on the stovetop. Carefully tilt the pan to one side and use a ladle to baste the butter over the breast of the turkey. Return the turkey to the oven, this time with the breast toward the back wall of the oven. Continue basting and rotating the turkey every 45 minutes until done. The turkey is done when an instant-read thermometer inserted into the thickest part of the thigh reads 165 degrees F, 2 1/2 to 3 hours total. If at any point during the cooking the skin of the breast becomes too dark, cover the breast with aluminum foil and continue cooking to doneness.
When the turkey is done, remove to a cooling rack to rest for 30 to 45 minutes.
To make the gravy, place the roasting pan on the stovetop. Turn the heat beneath the roasting pan to medium. Cook the vegetables, stirring constantly, until the butter remaining in the pan is translucent, up to 6 minutes. Pour off all but 2 tablespoons of the butter, if necessary. Sprinkle the flour over the vegetables and stir until a thick paste forms. Pour in the wine, stirring well and scraping the bottom of the pan to release any brown bits that remain stuck to the pan. Add the chicken stock and continue stirring until the stock reaches a simmer. Simmer for 10 to 15 minutes until flavorful and thickened to your liking.
Transfer the entire gravy mixture to a saucepot and bring back to a simmer. Whisk in the remaining 2 tablespoons butter. Season to taste with salt and pepper and strain the gravy into a serving container. Serve with the carved turkey.
Combine the salt, sugar, garlic powder, pepper, fennel and sage in an airtight container and shake to thoroughly combine. Keeps for up to 6 months.