Preheat the oven to 350 degrees. Place a turkey size oven bag in a roasting pan lined with a rack. Place the flour inside the bag.
Remove the turkey from the wrappings and place on a rimmed pan.
Season the butter pats generously with the seasoning salt. Slide the butter pats under the turkey skin, distributing them evenly around the turkey.
Season the outside of the bird generously with salt and pepper. Place the onion pieces inside the turkey cavity.
Place the turkey inside the prepared bag. Tie it shut with the enclosed silicone zip tie. Tuck the corners of the bag into the roasting pan.
Place the roasting pan into the heated oven and bake for several hours, according to the size of the turkey. Consult the time chart included with the oven bag. The turkey's internal temperature should be 165 degrees F.
Remove the turkey from the oven. Slice open the bag and remove the turkey to a serving platter or carving surface. Allow to rest at least 30 minutes.
Any drippings remaining in the bag or roasting pan are delicious for gravy, soups, and stocks. Drain and use for these purposes.
Carve the turkey and serve.
Notes
Cooked turkey is good in the refrigerator for up to 4 days, covered. It can be frozen for up to 6 weeks.Nutritional values are approximate and based on the size of the turkey.Variations: you can use other aromatics inside the turkey in addition or in place of the onion. These options would include leeks, carrots, and celery.Be sure to save the bones, drippings, and cooked aromatics to make homemade stock.