Spatchcock the turkey, and set aside. It's best to use an electric knife, but time and patience with a regular carving knife will work, too. Discard the organ meats, but keep the turkey neck and backbone. Place the backbone in a covered dish and refrigerate.
Preheat oven to 400 degrees F.
Make the marinade by placing the turkey neck in the bottom of a deep baking dish. Then, stir in the remaining marinade ingredients.
Bake the marinade for 30 minutes. Then, remove from oven and let come down to room temperature.
Pour the marinade into a very big heavy plastic bowl or baking dish. Pour the cooled marinade into the bowl. Then, place the spatchcock turkey breast-side down in the marinade. Season the exposed side of the turkey liberally with salt, black pepper and cayenne pepper.
Cover the turkey with multiple sheets of aluminum foil. Marinate the turkey overnight. The next morning, very carefully turn the turkey over so that the backside gets to sit in the marinade for a few hours. (It's totally ok if the turkey won't fit that way in your bowl. The legs and thighs will get plenty of seasoned juices released over them while roasting, so don't get discouraged if you can't marinade the other side of the turkey for some reason.)
Preheat oven to 450 degrees F.
Make the grain free and gluten free turkey gravy by placing the backbone in a deep baking dish. Cover the backbone with the chicken or turkey broth, then add the broth and remaining gravy ingredients to the dish. Stir until well-incorporated.
Grease a large sheet pan with butter or oil.
Spread the cut-up onion, bell pepper and celery over the greased sheet pan.
Place the spatchcock turkey, breast-side up over the veggies, on the top rack of the oven.
Bake for 20 minutes to brown the top of the turkey.
Turn the heat down to 350 degrees F. Slide the baking dish with the turkey gravy ingredients onto the bottom rack of the oven.
Cover the turkey and veggies loosely with foil, then cook for about 2 hours, or until a thermometer inserted into the breast and thigh reads 165 degrees F.
Remove the turkey gravy pan. Stir 1 tablespoon of tapioca starch or arrowroot starch into 3 tablespoons of cold water. Pour into the pan, and stir until well-incorporated. If the gravy isn't thick enough for your tastes, repeat the thickening process until satisfied.
Carefully remove the herb-roasted spatchcock turkey from the pan onto a large cutting board. Then, carve the turkey. Pour the pan drippings with veggies into a serving bowl.
Let the turkey rest for at least 15 minutes before serving. Then, serve with both gluten free turkey gravy and au jus with Cajun trinity veggies.
It took 2 hours to cook my 14-pound turkey. It may take more or less time, depending on the size of your bird and your oven. Please rotate the bird at least once every 30 minutes to ensure that it cooks evenly. And please, please use a digital thermometer to make sure that you cook the spatchcock turkey to the safe eating temperature of 165 degrees.