Cooking Turducken is part poultry anatomy lesson and part test of strength. How does one stuff a chicken inside a duck and then stuff that chicken-stuffed-duck inside a turkey? By ripping out most of the bones. Other than that, the process is far less complicated than you might think.
You'll want to use stuffing to fill in between the layers of poultry and to help baste from the inside. I use this recipe, which my wife found a few years ago while surfing the web. It may sound like a bit much, but it's especially useful in this case because it helps keep everything moist. You can skip the liver, but I'd suggest using an entire stick of butter.
I brined all three birds to keep things juicy and to reduce cook time. I used a very basic brine recipe. The proportions are one cup of kosher salt and one cup of light brown sugar per gallon of water. You'll want to soak the birds in the brine overnight. The chicken (about a three-pounder) and the duck (about a six-pounder) will fit in pots that you can fit in your fridge. The turkey (get a 20-pounder to hold all of the other stuff) will have to go in an ice chest, so don't forget to add some ice to simulate the temperature in your fridge (about 37°).
You'll want to watch a few YouTube videos to learn how to debone the birds. These are a lot like the YouTube videos that teach you how to tie a bow tie. You'll be frustrated at first that you can't do it as easily as the person in the video, but after a while you'll figure it out.
The mistake I made in assembling the Turducken was believing it could be held together with skewers. You should plan to sew the cavity shut with twine. If you have a huge sewing needle, use it. Trust me. It will make everything easier.
I roasted the Turducken at 500° for 15 minutes to get the skin crispy, and then I dropped the temperature to 225° and figured I'd go low and slow the rest of the way to preserve some moisture. That was a bad idea. You're looking for an internal temperature (in the chicken layer) of 155°. My Turducken plateaued, so I bumped the temperature up to 350° to finish the cook. You'd be better off just dropping the temperature from 500° to 325°. My cook took five and a half hours. I'm guessing it would have taken about three and a half and produced similar results with the higher temperature.
There's some serious effort involved in yanking out those bones, but it's absolutely worth it when you bite down into the finished product.
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