Start by removing thawed turkey from its packaging and remove giblets from body and neck cavities; discard the liver.
Place giblets in a stockpot and cover with 2 quarts of water.
Add onion, celery, salt, and pepper. Cover and cook on low for 3 hours.
To make the dressing:
Melt the butter in a large skillet and sauté onion and celery until transparent.
In a large pan or bowl, combine onion and celery mixture, herb stuffing, crumbled cornbread, cornbread stuffing, parsley, poultry seasoning, salt, and pepper.
Slowly add warm stock (excluding giblets and vegetables) in ¼-cup increments until the mixture becomes sticky and clumps on a spoon.
If more stock is needed, utilize chicken broth.
To assemble turkey:
Dry the inside and outside with paper towels and season the inside cavities with salt and pepper.
Stuff the body with dressing mixture and use a large craft needle and thread to sew closed.
Repeat with the neck cavity and tail.
Place the turkey on a rack in a roasting pan, position wings, and tie legs together.
Rub with vegetable oil and cover with a tent of heavy duty foil.
Cook for approximately 6 hours - one hour at 350° and then reduce heat to 325° for remaining time.
Make sure internal temperature reaches 165° and baste every 45 minutes.
When done, remove dressing from cavities as soon as possible to avoid spoilage, and allow turkey to rest for 30 minutes before serving.