Remove the giblets and neck from the turkey cavity. Place the turkey on a rack set in a roasting pan.
In a small bowl, combine the butter, thyme, and lemon zest.
Salt and pepper all over the outside of the turkey; also liberally salt and pepper inside of the turkey cavity and stuff with the lemon (that was zested for the butter mixture), onion, parsley, carrots and celery.
With your fingers, gently loosen the skin around the turkey breast. Grab tablespoon-size pieces of the butter mixture and place it in between the skin and the breast meat, going back as far as you can without ripping the skin. You won't use all the butter; there should be about half left over.
Once dots of butter are under the skin, press on top of the skin to mash the butter around into a relatively even layer; it definitely doesn't have to be perfect. You just want little pats of butter all over between the skin and the breast meat (see pictures below for a how-to).
Melt the rest of the butter. Brush it all over the skin of the turkey.
Roast the turkey for 2 1/2 to 3 hours, until the meatiest part of a chicken thigh registers a steady 165 degrees F.
Remove the turkey from the oven and let rest for 15-20 minutes before carving and serving.
Turkey: I know it might seem like "go big or go home" when it comes to turkey, but the bigger the turkey, the harder it is to cook it evenly and get a great, juicy, perfect result. If you can, follow the size guidelines as closely as possible (although I won't blame you if you get a 25-pounder and just go for it).Gloves: if raw poultry in general kind of creeps you out, feel free to don plastic gloves to do the whole butter-under-the-skin thing. It's a totally acceptable way to get the job done.