Set a rack inside a large heavy roasting pan. Season turkey lightly inside and out with salt and pepper, then with spice mix, massage it into skin. Transfer turkey, breast side down, to prepared pan and refrigerate, uncovered, overnight. Remove turkey from refrigerator; let stand at room temperature for 1 hour.
Heat oven to 375°F. In a medium bowl, mix celery, pepper, and onion. Fill turkey cavities with vegetable mixture, scattering any remaining vegetables over bottom of roasting pan. Brush turkey with oil.
Roast turkey, basting occasionally, for 1 hour. Using paper towels, flip turkey. Roast, basting occasionally, about 1 to 1 1/2 hours longer, until an instant-read thermometer inserted into thickest part of thigh registers 165°F. Transfer to a platter. Let rest for at least 20 minutes before carving. Serves 10.
Approximate Nutrient Analysis per serving (based on 12 pound turkey and no additional salt besides Cajun-Creole seasoning blend):
550 calories, 24 g fat, 5 g saturated fat, 260 mg cholesterol, greater than 2300 mg sodium, 9 g carbohydrate, 3 g fiber, 2 g sugar, 72 g protein