Preheat the oven to 425°F and line a half sheet pan or rectangle roasting pan with aluminum foil.
Slice the onion into half moons and place in a single layer on foiled line pan.
In a bowl, combine the very softened butter, parsley, rosemary, minced garlic, salt and black pepper. Stir it all together well.
Place the spatchcocked turkey on top of the onions.
Using your fingers separate the skin from the breasts. Put about a tablespoon of the butter under the skin of each breast, pushing the butter around to spread it out under the skin.
Then spread the remaining butter mix all over the top of the turkey including the legs and wings, by pushing the butter down and out in one direction. Using a small or mini rubber spatula works best.
Sprinkle a little extra salt and pepper over the top.
Roast the turkey in preheated oven for 30 minutes on center rack.
REDUCE heat to 375°F and baste the turkey with its juices.
Roast the turkey for another 60-90 minutes depending on size of turkey, and basting every 20 minutes.
Once the turkey has reached an internal temperature of 160°F remove the turkey from the oven and let rest uncovered for at least 20 minutes. To check the temperature place the thermometer in the thickest part of the breast while not touching any bones or in the thickest part of the thigh. I like to check multiple spots just to make sure.