Remove giblets and neck from turkey; place in refrigerator for use in gravy, if desired. Rinse turkey with cold water; pat dry with paper towels. Place turkey, breast side up, on a rack in a lightly greased roasting pan. Lift wing tips up and over back, and tuck under bird.
Combine butter and next 4 ingredients in a small bowl; rub 2 Tbsp. seasoned butter inside turkey cavity. Place onion, celery, and garlic inside turkey cavity. Rub remaining 4 Tbsp. seasoned butter all over outside of turkey, legs and all. Tie ends of legs together with heavy string, or tuck under flap of skin around tail.
Bake, uncovered, at 325° for 2 1/2 to 3 hours or until a meat thermometer inserted into the meaty part of thigh registers 170°. Shield turkey with aluminum foil towards end of cooking, if necessary, to prevent overbrowning.
Transfer turkey to a serving platter, reserving pan drippings for Bourbon Gravy. Let turkey stand, covered with foil, at least 15 minutes before carving. Garnish platter, if desired. Serve turkey with Bourbon Gravy.