Remove giblets and neck, and rinse turkey with cold water; pat dry. Sprinkle cavity with 1 teaspoon salt and 1 teaspoon pepper. Place turkey, breast side up, in a roasting pan. Lightly stuff turkey body cavity with coarsely chopped onion, celery, apple, and 3 rosemary sprigs. Rub turkey with butter.
Loosen skin from turkey breast without totally detaching skin; carefully slip bacon slices under skin. Replace skin, using wooden picks to fasten skin over breast meat. Tie ends of legs together with heavy string, or tuck under flap of skin. Lift wing tips up and over back, and tuck under bird. Pour 3/4 cup apple cider over turkey.
Bake turkey, uncovered, at 450° for 20 minutes. Reduce heat to 350°; bake 2 hours and 45 minutes or until a meat thermometer inserted into meaty part of thigh registers 180°, basting often with pan juices. Shield turkey with heavy-duty aluminum foil during cooking, if necessary, to prevent overbrowning.
Transfer turkey to a serving platter; cover loosely with foil. Scrape roasting pan to remove browned bits; pour pan drippings into a gravy separator. Discard fat, reserving 2 tablespoons.
Heat reserved 2 tablespoons fat in a small saucepan over medium-high heat. Add finely chopped onion, garlic, and 2 teaspoons rosemary; sauté 2 minutes or until tender. Reduce heat to medium; add flour, and cook 1 minute. Whisk in pan drippings and remaining 1/2 cup cider. Bring to a boil; reduce heat, and simmer 2 to 3 minutes or until gravy thickens. Stir in 1/4 teaspoon salt and 1/4 teaspoon pepper. Serve gravy with turkey. Garnish, if desired.
Note: For ease in removing hot turkey from roasting pan, wear clean heat-resistant rubber gloves. They shield the heat and enable you to get a firm grip on the bird.