Melt the butter in a small saucepan. Add the zest and juice of the lemon and 1 teaspoon of thyme leaves to the butter mixture. Set aside.
Place the turkey in a large roasting pan. Liberally salt and pepper the inside of the turkey cavity. Stuff the cavity with the bunch of thyme, halved lemon, quartered onion, and the garlic. Brush the outside of the turkey with the butter mixture and sprinkle with salt and pepper. Tie the legs together with string and tuck the wing tips under the body of the turkey.
Roast the turkey until it reaches 170F or until the juices run clear when you cut between the leg and the thigh. Remove the turkey to a cutting board and cover with aluminum foil; let rest for 20 minutes.