Rinse the turkey in clean water and pat dry with paper towels. Remove the gizzards and neck.
Place gizzards and neck into a pan of water and bring to a simmer to create broth for gravy. Simmer for 30 to 45 minutes. Remove from the heat, strain, and reserve.
Meanwhile, make cornbread from mix, according to package directions. Crumble cornbread into small pieces and lay onto a cookie sheet to air dry.
In a bowl, combine celery and green onions with air-dried cornbread. Add poultry seasoning and soup, but no water. Mix together thoroughly and place inside turkey cavities.
Generously cover bird completely with softened butter, tent with aluminum foil and bake according to package instructions for bird's weight. One hour before bird is finished cooking, remove foil and baste. This will allow skin to become crispy and brown without burning.
The turkey is done when the internal temperature registers 170 degrees F deep in the thigh, and juices run clear when flesh is pierced. The temperature of the stuffing should be 160 degrees F in the center.
Remove bird from oven, place on serving platter, and tent with foil. Let turkey rest for about 15 minutes so the temperature of the stuffing will reach 165 in the center and the temperature in the thigh will rise to 180 degrees F.
Start gravy. In a large skillet, heat the vegetable oil, add the flour and brown deeply, stirring constantly over medium-low heat. Add turkey drippings and whisk until thickened. Add gizzard broth and simmer. Season with salt and pepper, to taste.
Remove stuffing from bird and place in serving bowl. Carve turkey and plate with stuffing and gravy.