Preheat the oven to 400 degrees F. Place an oven rack one spot down from the center, leaving the space above it open.
In a small saucepan over medium-low heat, combine the butter, garlic, lemon zest, and rosemary. Cook until the butter is melted. Remove from the heat and set aside.
Remove the turkey from the package, discard the neck and giblets, and pat it dry with paper towels. Place the turkey onto the roasting pan rack. Pour the lemon juice all over. Brush the butter mixture all over the turkey skin, then sprinkle it generously with salt and pepper.
Pour the chicken broth into the bottom of the roasting pan. Stuff the lemon halves into the turkey cavity.
Roast for 1 hour. Then turn the heat down to 375 degrees F. Baste the turkey about every 30 minutes while it cooks. Once the breast reaches 160 degrees F and the thigh is around 175 degrees F, the turkey is done.
Remove the roasting pan from the oven. Transfer the turkey to a serving platter and let it rest for 20 minutes before carving. Garnish the platter with lemon slices and rosemary sprigs if desired.
Notes
This recipe works best with a 15 to 18-pound turkey. Cooking time will vary based on the size of your bird. Plan on 1.5 pounds of turkey per guest.