Preheat oven to 325° with rack in lower third of oven. Remove giblets and neck. Rinse giblets and place in a 3- to 4-qt. saucepan. Add 1 cup each of onion and celery, next 2 ingredients, 1 tsp. salt, and 1/2 qt. water. Bring to a boil over high heat. Reduce heat to low; partially cover, and simmer 1 1/2 hours. Strain broth to measure 1 qt., adding water if necessary. Discard solids.
While broth simmers, toss together remaining onion and celery in a bowl. Rinse turkey, and pat dry with paper towels. Spray broiler pan and both sides of rack with vegetable cooking spray. Sprinkle body and neck cavities of turkey with 1/2 tsp. salt and 1/4 tsp. pepper. Sprinkle 1/4 tsp. garlic powder in body cavity.
Wrap 1 1/2 cups onion mixture in 2 layers of cheesecloth. Saturate cheesecloth packet with water; squeeze out excess moisture, and place in body cavity. Repeat procedure with remaining 1/2 cup onion mixture; place in neck cavity. Cover packet with neck skin, folding wing tips under to secure. Tie legs together with kitchen string.
Place turkey, breast side up, on prepared broiler pan. Sprinkle bird with 1/2 tsp. salt, remaining 1/2 tsp. pepper, and remaining 1 tsp. garlic powder, and rub evenly over skin. Brush bird with canola oil. Bake at 325° for 15 minutes per pound or until a meat thermometer inserted in thigh registers 180° and drumstick is soft to the touch and leg moves easily (3 to 3 1/4 hours).
Remove turkey from pan, and place on a platter, reserving drippings in pan. Cover turkey with foil, and let rest 45 minutes before carving.
Meanwhile, prepare gravy, using a rubber spatula to scrape drippings, including all fat and dark brown particles, into a 2-cup liquid measuring cup. (Do not use any bitter charred particles.) Allow fat to rise to top of drippings. Pour 1/3 cup fat into a 3-qt. saucepan. Carefully drain remaining fat, reserving all brown drippings (about 1 1/4 cups). Whisk flour into fat in saucepan until smooth. Whisk in brown drippings. Cook, whisking constantly, over medium-high heat until bubbly. Whisk in strained broth. Bring to a boil, whisking constantly; reduce heat to medium. Cook, uncovered, 10 minutes or until desired consistency, stirring often. (Gravy will thicken as it cools.)
Remove and discard packets from turkey. Carve each side of turkey breast from turkey in one piece. Cut breast halves crosswise into 1/4-inch slices. Carve remaining turkey. Serve with gravy. Make ahead: Cook, strain, and chill broth a day ahead. Prepare turkey for baking (do not brush with oil), and place on a jellyroll pan. Refrigerate, uncovered, overnight. Transfer turkey to prepared broiler pan. Brush bird with oil, and bake as directed. (Refrigerating the bird uncovered produces an extra crispy skin.)
This recipe is excerpted with permission from <a href="http://www.amazon.com/gp/product/0848739663/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0848739663&linkCode=as2&tag=craftfocom-20">"Christmas with Southern Living 2013: The Ultimate Guide to Holiday Cooking & Decorating"</a> by the Editors of Southern Living Magazine and published by Oxmoor House.