Wash turkey and pat dry with paper towel. Fasten neck skin to back with toothpicks or skewer. Fold wings across back with tips touching.
Combine onion, carrot, celery, and 3 cloves garlic in bowl.
Combine seasoning salt, thyme, rosemary, garlic powder, sage, pepper and cayenne. Rub turkey cavity with half of the seasoning mixture. Fill cavity with vegetable mixture. Tuck drumstick under band of skin at tail or tie together with heavy string. Place breast side up on rack in shallow roasting pan. Brush turkey with olive oil or melted butter.
Sprinkle seasoning over entire turkey. Insert meat thermometer so the tip is in the thickest part of inside thigh muscle or the thickest part of breast meat without touching bone. Roast the turkey at 325 degrees F.
The most reliable method for determining when a turkey is done is by using a meat thermometer. The turkey is done when the temperature reaches 180 degrees F to 185 degrees F in thigh (juices should be clear, not pink, when thigh muscle is pierced deeply); 170 degrees F to 175 degrees F in the thickest part of the breast, just above the rib bones.
Let the turkey rest for 15 to 20 minutes before carving to allow juices to distribute evenly.