Preheat the oven to 350º Fahrenheit. You'll place the turkey on the lowest roasting rack so make sure you have your oven prepared for that before it gets hot.
Place the flour inside the oven bag and shake it to coat the bag.
Remove the giblets and neck from the turkey. Discard or save for gravy or another use.
Place the turkey inside the bag. Place the turkey in a roasting pan.
Place a few pieces of the onion, carrots and celery inside the turkey, then place the rest around the turkey.
Cut the stick of butter in tablespoon slices (8 slices).
Loosen the skin of the turkey from the breast.
Slide the butter slabs under the skin of the turkey breast. We do 4 slices on each side.
Sprinkle the turkey with salt and pepper if desired.
Seal the turkey bag with the tie. Be sure to have the bag loose around the turkey.
Use a sharp knife to cut 3 slits in the top of the bag.
Roast the turkey until the temperature measures a minimum of 165º Fahrenheit in the innermost part of the thigh and wing and the thickest part of the breast. (We like to cook ours just a little bit longer than this.)
The exact time will depend on how big the turkey is, and if you've stuffed the turkey or not. Plan for 3-4 hours for an 18 pound unstuffed turkey. Adjust timing from there.
Let the turkey stand for 15 minutes after removing from the oven.
Then cut slits in the top of the bag to allow any steam to release.
Use the pan drippings for gravy, then place on a serving platter, slice and serve.
Add seasonings such as garlic powder, onion powder, thyme, rosemary, garlic or paprika to the outside of the turkey if you'd like.