Preheat the oven to 350°F
Place an oven-safe roasting bag in a large roasting pan
Add 1 tablespoon flour to the bag and shake to coat the inside
Pat the turkey dry
Season the turkey as desired
Place chopped onions, carrots, and celery in the bag if desired
Put the turkey into the bag
Add butter or oil on top of the turkey if desired
Seal the bag with the provided tie
Cut 4 to 6 small slits in the top of the bag
Tuck the ends of the bag into the pan
Roast the turkey until fully cooked
Check the internal temperature in the thickest part of the thigh and breast
Remove the turkey when it reaches 165°F
Let the turkey rest for 20 to 30 minutes before carving