Preheat the oven to 350°F with the top rack in the lower third of the oven.
Remove the neck and the bag of innards and giblets from inside the cavities. Sometimes they’re both in the large main cavity, but sometimes the bag is in the smaller neck cavity around the other side.
If there are any pin feathers still on the bird, pluck them out.
Place the turkey breast-side up into a large roasting pan. No racks or fancy positions necessary.
Place butter into a small bowl. Strip one teaspoon of leaves from the thyme and add it to the bowl, reserving the rest. Add the salt, pepper, and lemon zest, and mix it all together thoroughly with a fork.
Use your fingers to carefully separate the turkey skin from the breast meat to create pockets. Tuck some of the compound butter into the pockets and distribute it between the skin and the breast meat by wriggling it in as best as possible. Spread the remaining compound butter all over the outside of the turkey.
Place the lemon and onion pieces into the large cavity of the bird along with a the remaining thyme sprigs.
To truss, tuck the tip of each wing under the bird and tie the tips of the drumsticks together with a small piece of twine if this isn’t already done.
Place turkey into oven and roast to your desired level of doneness. For me this means that an instant read thermometer inserted into the deepest part of the breast should read 155-160°F. Refer to the notes section below to learn more. A 10 pound turkey will probably need between 2 and 2 ½ hours. A 14 pound turkey will probably need between 2 ½ and 3 hours.
Remove turkey from oven and let rest for at least 30 minutes before carving.
How to make gravy from pan drippingsIf you like, you can make gravy right in the roasting pan while the turkey rests. Here's what to do.
Set the roasting pan with the drippings over medium heat on the stovetop. Depending on your stove and the size of the pan, sometimes it's best to do this over two burners, sometimes one.
Estimate the amount of drippings in the pan. If it seems vaguely like ½ cup, you're all set. If it seems like a lot more, spoon some out. If it seems like a lot less, add some butter and let it melt.
Sprinkle in ½ cup flour and stir vigorously with a sturdy whisk or a wooden spoon, scraping up browned bits from the bottom of the pan and incorporating the flour into the fat to form a roux. Cook for a minute or so, stirring constantly.
Pour in 8 cups chicken broth or turkey stock (or seven cups stock and one cup dry white wine). Bring to a boil, stirring constantly.
Reduce heat and simmer, stirring frequently, until thickened to your liking.
Taste for seasoning and then ladle into a gravy boat to serve.