Preheat the oven to 350 F degrees.
In a small saucepan melt the butter over medium heat. Add the lemon zest and juice and the teaspoon of thyme to the butter and stir. Set aside.
Wash the turkey and make sure you remove the giblets from inside the turkey cavity. I also usually trim some of the excess fat around the neck of the turkey.
Place the turkey in a large roasting pan with a roasting rack. Salt and pepper the inside of the turkey cavity. Stuff the turkey with the onion, garlic and thyme.
Brush the turkey with the butter mixture all over and generously season with salt and pepper.
Tuck the wing tips under the body of the turkey, this will help stabilize the turkey when carving, plus it makes it easier to carve the breast. Tie the legs together with butcher twine.
Cover the turkey with the lid, or if your roasting pan doesn't have a lid cover with aluminum foil.
Transfer the roasting pan to the oven and roast the turkey for 2 hours, remove lid or aluminum foil. Roast for an additional 1 1/2 hours. Use a meat thermometer to know when the turkey is done. When the thermometer is inserted in the breast it should read 165 F degrees, and 180 F degrees in the thigh.
Remove the turkey from the roasting pan to a cutting board and cover with aluminum foil. Let it rest for 20 minutes.
Slice turkey and serve.