Prepare the brine according to recipe directions. Brine the turkey for 18-24 hours before cooking.
Preheat the oven to 425 degrees F.
Remove the turkey from the brine; rinse and pat dry. Let the turkey stand at room temperature for 20 minutes. Tuck the turkey wings under the body.
Place the onion, celery and bell pepper into the cavity of the turkey. Tie the turkey legs together with kitchen twine.
Place the Cajun seasoning and butter in a bowl and stir until well combined.
Rub the Cajun spice butter all over the outside of the turkey and under the skin of the breast.
Place the turkey in a roasting pan and bake for 30 minutes.
Reduce the oven temperature to 350 degrees F and continue roasting, basting the turkey every 30 minutes with pan drippings, until the turkey is done.
This process will take approximately 3 1/2 hours and you'll know that the turkey is done when a meat thermometer inserted into the thickest part of the turkey thigh or breast registers at least 165 degrees F.
If the turkey seems like it's getting overly browned during the cooking process, cover it with foil.
Remove the turkey from the oven and tent loosely with foil. Let the turkey rest for 10 minutes. Slice, then serve.
Garnish with fruit and herbs if desired.