Cut butter into 1cm pieces and chill until needed.
Use your fingers to loosen the skin over the turkey breast and thighs. Cut a hole in the turkey skin between tail and body (to affix legs); cut a hole on each side of the turkey beneath each wing (to affix wings). Rub the entire skin with vegetable oil.
Toss apple and onion wedges, garlic cloves and celery in a large bowl until combined; sprinkle poultry seasoning over the apple mixture and toss again to coat. Stuff turkey with the apple mixture. Insert each wing tip into the hole under the wing to secure wings; insert both legs into hole near tail to secure legs.
Place turkey onto a rack and set in a roasting tin. Insert cold butter pieces under the loosened skin, distributing the butter as evenly as possible.
Roast the turkey in the preheated oven until an instant-read meat thermometer inserted into the thickest part of a thigh, not touching bone, reads 70 degrees C, about 3 to 3 1/2 hours. Check for doneness after 3 hours.
Remove roasting tin from oven and cover turkey with aluminium foil, lightly pressing the foil directly onto the bird. Let the turkey rest for 40 minutes before carving.
Carve the entire turkey and save the bones, joints, tail, wing tips, dripping and reserved onion, garlic and celery to make turkey soup.