Shake flour in oven bag, then place bag in a large roasting pan at least two inches deep. To reduce having the turkey stick to the bag, spray inside of the bag with nonstick spray.
Chop onions, celery and carrots, and place in oven bag. Rub softened butter liberally all over turkey. Sprinkle salt, garlic pepper, thyme and sage all over turkey (inside too)!
Place turkey in oven bag on top of vegetables. Close oven bag with nylon tie, then cut six ½-inch slits in top. I recommend inserting a meat thermometer through slit in bag into the turkey to monitor cooking temperature. Tuck ends of bag in the pan, ensuring the bag is not overhanging the pan.
Bake approximately 2½ to 3 hours or until meat thermometer reads 165 degrees Fahrenheit. (Note: If needed, atonce it is at this temperature but not browned on top, I carefully split the bag and turn the oven to broil to brown the top of the turkey to a beautiful, golden color. If you do this step, be sure to not walk away from the oven, it can brown very quickly.)
Allow turkey to stand for 15 minutes before carving. Reserve bag liquid for gravy.