Preheat oven to 425°. Remove giblets and neck from turkey, reserving neck. Rinse turkey with cold water; pat dry. Sprinkle outside of turkey and cavity with salt and 1 tsp. pepper; stuff cavity with apples, half of onion, and 6 rosemary sprigs. Loosen skin from turkey breast without totally detaching skin; carefully place sage and remaining 2 rosemary sprigs under skin. Replace skin. Place turkey, breast side up, on a rack in a roasting pan.
Inject 1 1/2 cups Apple-Bourbon Marinade into turkey breast, thighs, and drumsticks.
Bake at 425° for 20 minutes; reduce oven temperature to 350°, and bake 4 hours or until a meat thermometer inserted into thickest portion of thigh registers 180°, shielding with aluminum foil to prevent excessive browning, if necessary. Remove turkey from pan, reserving 1/2 cup drippings. Let turkey cool 30 minutes. Remove onion, apple, and rosemary from cavity, and discard.
Bring reserved turkey neck, 2 cups water, and remaining Apple-Bourbon Marinade and onion half to a boil in a saucepan; boil 15 minutes. Pour through a wire-mesh strainer into a measuring cup, discarding solids.
Heat reserved 1/2 cup drippings in a heavy saucepan over medium heat; whisk in flour until smooth. Cook, whisking constantly, 1 minute. Gradually add apple-bourbon mixture, whisking constantly. Cook, whisking constantly, 10 minutes or until thickened. Stir in remaining 1/2 tsp. pepper. Serve warm with turkey.
Note: Turkey may be basted with Apple-Bourbon Marinade every 30 minutes during baking instead of injecting with marinade.