Why Spatchcock a Turkey?
by J. Kenji López-Alt, the Chief Culinary Advisor of Serious Eats (full article on Serious Eats)
Advantage 1: Flat Shape = Even Cooking
By laying the bird out flat and spreading the legs out to the sides, what was once the most protected part of the bird (the thighs and drumsticks) are not the most exposed. This means that they cook faster—precisely what you want when your goal is cooking the dark meat to a higher temperature than light meat.
As a bonus, it doesn't take up nearly as much vertical space in your oven, which means that if you wanted to, you could even cook two birds at once. This is a much better strategy for moist meat than trying to cook one massive bird.
Advantage 2: All Skin on Top = Juicier Meat and Crisper Skin
A regular turkey can be approximated as a sphere with meat inside and skin on the outside. Because it's resting on top of a roasting pan or baking sheet, one side of that sphere will always cook more than the other.
A spatchcocked turkey, on the other hand, resembles a cuboid, in which the top surface is skin and the volume is meat. This leads to three end results. First, all of the skin is exposed to the full heat of the oven at the same time. There is no skin hiding underneath, no underbelly to worry about. Secondly, there is ample room for rendering fat to drip out from under the skin and into the pan below. This makes for skin that ends up thinner and crisper in the end.
Finally, all of that dripping fat bastes the meat as it cooks, helping it to cook more evenly, and creating a temperature buffer, protecting the meat from drying out.
Advantage 3: Thinner Profile = Faster Cooking
A normal roast turkey can take several hours to cook through at an oven temperature of around 350°F or so. Try and increase that heat, and you end up scorching the skin before the meat has had a chance to cook through.
With a spatchcocked turkey and its slim profile, this is not a problem. You can blast it at 450°F and it'll cook through in about 80 minutes without even burning the skin. In fact, you want to cook it at this temperature to ensure that the legs and breasts end up cooking at the same time (lower heat leads to a lower differential in the internal temperature between hot and cool spots), and that the skin crisps up properly.
It's a time savings of about 50%! If I added up all the time I could have saved in Thanksgivings past using this method, I could perhaps—dare I say it—rule the world?
Advantage 4: Turkey Backbones = Better Gravy
It's always possible to make gravy with nothing but canned chicken stock and drippings, but that gravy is so much better when you have some real bones and meat to work with. Normally, that means using the turkey neck and giblets to flavor the broth while the turkey roasts. You can still do that. But this time, you can add the entire turkey's back to the mix, resulting in a far more flavorful broth with which to make your gravy.