How to Make It
Remove giblets and neck from turkey; discard or reserve for another use. Trim excess fat from turkey; pat turkey dry. Place turkey, breast side up, on a rack set inside a rimmed baking sheet. Place, uncovered, in refrigerator. Combine stock, butter, 6 thyme sprigs, 3 rosemary sprigs, and smashed garlic cloves in a small saucepan. Cook over low, stirring occasionally, until butter has melted and flavors are infused, about 10 minutes. Remove from heat. Let stand at room temperature 2 hours. Cover and place in refrigerator. Let turkey and butter mixture chill 8 hours or overnight.
Remove turkey from refrigerator. Let stand at room temperature 1 hour; discard any accumulated juices on baking sheet. Preheat oven to 500°F with oven rack in lowest position. Remove butter mixture from refrigerator; reheat over low, stirring occasionally, until melted and warm, about 8 minutes. Strain into a small bowl; discard solids. Add salt and sugar to strained butter mixture, and stir until dissolved. Add zest. Place halved onion, halved lemon, halved garlic head, remaining 6 thyme sprigs, and remaining 3 rosemary sprigs inside body cavity of turkey. Secure legs with kitchen twine. Using a turkey injector, inject butter mixture throughout breast, legs, and thighs.
Roast turkey at 500°F until skin is golden, 35 to 40 minutes. Remove from oven, and cover breast with foil. Reduce oven temperature to 350°F. Return turkey to oven immediately, and bake until a thermometer inserted in thickest part of breast registers 160°F, about 1 hour and 20 minutes. Transfer turkey to a cutting board. Let turkey rest, loosely covered with foil, 15 minutes (internal temperature will rise to 165°F).