Rinse your turkey, and pat dry inside & out. If you want to brine your bird and/or inject it, go ahead and do those steps first.

Preheat your oven to 275 degrees.

Combine the softened butter, chopped parsley, minced garlic, seasoning salt, onion powder, & black pepper in a small bowl. (I like using my hands for this!). Place a spatula or your hand in between the skin on the breast & the meat. Coat under the skin with the herb butter. Using your hands, spread the rest of the herb butter on top of the turkey's skin & inside the cavity as well. Sprinkle with Cajun seasoning (or seasoning of your choice)

Tie the turkey legs together with cooking twine - or use the extra skin to hold the legs together (like in the pictures in the post). Stuff the inside of the cavity with the onion quarters & celery halves. Tent a piece of tinfoil on top of the breasts area. Roast the turkey in a roasting pan on a roasting rack.