1Heat oven to 325°F. Grease 13x9-inch (3-quart) glass baking dish with shortening or cooking spray. In 8-inch skillet, melt butter over medium heat. Add onion; cook 4 to 6 minutes, stirring occasionally, until tender.
2In large bowl, thoroughly mix cooked onion and remaining ingredients. Use 4 cups of the stuffing to fill body cavities of 12-pound turkey (do not pack stuffing because it will expand during roasting). Place remaining stuffing in baking dish. Cover with foil; refrigerate until ready to bake.
3Roast stuffed turkey as directed in recipe. Bake stuffing in baking dish with turkey for the last 40 to 50 minutes of baking time or until thoroughly heated (165°F).