In my books, the perfect bird is 14 to 18 pounds/ 6.5 kg to 8 kg in weight because that's a good size to handle, feeds about 10 to 14 people and has better flavour than bigger birds. If you're buying from a small producer, like the lovely turkey I used from my mate Paul Kelly, you'll often find these birds come with their own cooking instructions. Really good-quality birds do cook in a shorter time so follow the instructions if it has them.
This year I'm using a flavoured butter to give a bit of extra love to my turkey, and this is a job you can do the day before. Get your turkey and use a spoon to work your way between the skin and the meat. Start at the side of the cavity just above the leg and work gently up towards the breastbone and towards the back so you create a large cavity. Pick up half of your butter and push it into the cavity you've created. Use your hands to push it through the skin right to the back so it coats the breast meat as evenly as possible. Do the same on the other side then rub any leftover butter all over the outside of the bird to use it up. If you've got any herb stalks left over, put them in the cavity of the turkey for added flavour as it cooks. Cover the turkey in cling film/ plastic wrap and keep in the refrigerator until you need it.
Take your turkey out of the refrigerator a few hours before you are ready to put it in the oven so it has time to come up to room temperature. That flavoured butter will already be under the skin so you'll only need a few tweaks to finish it off. Halve 2 to 4 clementines and pop them in the cavity with a few more sprigs of fresh herbs like rosemary, bay and thyme. The fruit will steam and flavour the birds in a really lovely way. Take a sprig of fresh rosemary, pull off the leaves at the bottom then spear that through the loose skin around the cavity to hold it together and keep it from shrinking back as the turkey cooks.
Open up the neck cavity and pack as much stuffing as possible in there, then carefully pull the skin back over the cavity, tuck it under the bird and pop it in the roasting tray. If you've already made your gravy like I've done, you won't need a vegetable trivet, if not, do that now by roughly chopping 2 or 3 carrots, 3 peeled onions, and 2 celery sticks. Preheat your oven to full whack and get the turkey in the roasting tray. As soon as it goes in the oven, immediately turn the heat down to 350 degrees F/ 180 degrees C/ Gas 4.
As a rough guide, you want to cook the turkey for about 35 to 40 minutes per 2.2 pounds or 1 kilogram, so a 15 1/2 pound/ 7 kg turkey will want about 4 to 4 1/2 hours in the oven. But there are so many variables such as the sort of oven you have and the quality of your bird. Check on your turkey every 30 minutes or so and keep it from drying out by basting it with the lovely juices from the bottom of the pan.
After 3 1/2 hours, remove the foil so the skin gets golden and crispy. If you are at all worried just stick a meat thermometer in the thickest part of the breast. When the internal temperature has reached 150 degrees F/ 65 degrees C for a good quality bird, and about 180 degrees/ 82 degrees C for a cheaper bird, it's ready to come out.
Carefully put a metal skewer in the cavity and use it to lift the bird and angle it over the roasting tray so all of the juices from the cavity run out. Move the turkey to a platter then cover it with a double layer of tinfoil and 2 tea towels to keep it warm while it rests for at least 30 minutes.