To achieve the crispy skin your first goal should be to get the surface as dry as possible before hitting the oven. This can be achieved by rubbing the skin with good quality sea salt the night before. Adding a small amount of baking powder to the mixture will also help. This process will help to dry the skin and will also help with tenderising of the meat. As an alternative you could wet brine your turkey if you have a large enough non reactive container.
To help with crisping the skin we need a good airflow around the bird when it's in the oven, this will take more moisture from the skin to allow it to crisp better.
The next problem to solve is the moist succulent meat in both parts of the bird. The breast of the turkey is comprised of muscles that do very little work and are designed to give a burst of power during takeoff. These muscles are lean and tender. In contrast the thigh and leg muscles are carrying the weight of the bird and in constant use while it's active. For this reason they have more fat and connective tissue and are tougher. Ideally we would cook the breast separately from the rest of the bird… a good reason why many people opt for a crown.
In an ideal world we would want the breast to cook to an internal temperature of around 65c and the legs/thighs to reach a much higher 75c, or higher, to breakdown the connective tissue.
After some internet research and a bit of experimentation, I think I may have come up with a way of getting to the desired result without separating the crown from the rest of the bird.
The problem, is the conventional roasting tin that many of us go out and buy once a year to roast a ridiculously large bird. The roasting tin stops the airflow we're looking for meaning that we end up with wonderful crispy skin on the top of the bird and flabby pale skin underneath. We're also protecting the thighs and legs from the heat by sitting them inside a pan. This is the opposite to the goal we set out with.
I've tried roasting the turkey upside down and then turning it over half way through cooking. This works, but it's not ideal. First of all, it's not that easy with a large hot bird (sorry couldn't resist that) and secondly you risk damaging the breast skin if you miss time it.
The answer is to replace the deep sided roasting tin with a shallow lipped baking tray. This will expose the legs and thighs to the heat from the oven. The next step is to use one of those V shaped wire racks to sit the bird on. This will raise it above the surface of the tray and allow the air to circulate under the turkey, avoiding the soggy bottom, to allow the skin to crisp all round.
We've still got the problem of the breast cooking before the legs and thighs. I found an answer on the internet in the form of a baking steel, something I'd never heard of before. This is a thick piece of steel that you can put in your oven for baking bread and pizza etc. The baking steel holds large amounts of heat energy that can be released into the bottom of your turkey. I've tested this and discovered that a cast iron griddle works almost as well, if you're really stuck it will also work with a heavy cast iron frying pan turned upside down!
The technique is to put the baking steel in the bottom of your oven or on a rack low down and heat up the oven as hot as it will go. Heat for around 30-40 minutes to get a good amount of heat into the steel and the oven then place your racked up turkey in the baking tray directly on top of the steel. Turn down the oven to around 150°C and close the door. The sheet will steadily release heat to the underside of the turkey while the air in the oven circulates around the rest of the bird. The initial heat of the oven should brown and crisp up the skin before cooling and gently cooking the breast meat.
It's amazing what you can find on the internet if you look hard enough, just be careful of the search terms you use, as it all goes into your browsing history and you might get some very strange adverts!