Preheat oven to 325°F.

Remove neck, heart, gizzard, and liver from turkey. Reserve neck, heart, and gizzard for Homemade Turkey Stock.

Trim excess fat from onion, celery, 1 tablespoon rosemary, ½ teaspoon salt, and ¼ teaspoon pepper. Stuff turkey cavity with onion mixture.

Tie ends of legs with twine. Lift wing tips up and over back; tuck under bird.

Place a roasting rack coated with cooking spray in a roasting pan. Place turkey, breast side up, on rack.

Brush with butter; sprinkle evenly with 1 teaspoon salt and ¼ teaspoon pepper.

Pour ⅓ cup Homemade Turkey Stock into bottom of pan. Insert a meat thermometer into meaty part of thigh, making sure not to touch bone.

Cover turkey breast tightly with foil. Bake at 325°F for 2 hours, basting with ⅓ cup Homemade Turkey Stock every 30 minutes.

Remove foil; bake an additional 1 ½ hours or until thermometer registers 165°F, basting every 30 minutes with ⅔ cup total.

Remove turkey from oven; let stand 20 minutes. Discard skin.

Place a drip-top plastic bag inside a 2-cup glass measure. Pour pan drippings into bag; let stand 10 minutes.

Seal bag, and snip off 1 bottom corner of bag. Drain drippings into a measuring cup, stopping before the fat layer reaches the opening. Reserve 2 tablespoons fat.

Heat 2 tablespoons reserved fat in bottom of roasting pan over medium heat. Add flour, stirring with a whisk. Cook 1 minute, stirring constantly.

Stir in pan drippings mixture. Combine water and cornstarch, stirring with a whisk. Add cornstarch mixture to pan, stirring with a whisk.

Bring to a boil, stirring frequently. Add 1 tablespoon rosemary. Reduce heat, and simmer 5 minutes or until sauce thickens.

Stir in ¼ teaspoon salt and ¼ teaspoon pepper. Serve gravy with turkey.