How to Make It
Combine 8 cups water, salt, sugar, peppercorns, garlic, lemon, thyme sprigs, rosemary sprigs, and bay leaves in a large stockpot. Bring to a boil over high, stirring to dissolve salt and sugar. Remove from heat. Add ice, and whisk until ice melts. Add chicken. Place a plate upside down on top of chicken to submerge completely. Cover and chill 12 hours.
Preheat oven to 375°F. Cut top from garlic head to just expose cloves (about 1/2 inch). Drizzle exposed top with oil. Wrap garlic head in aluminum foil. Place directly on rack in preheated oven, and roast until cloves are tender and creamy, about 40 minutes. Carefully open foil packet, and let cool slightly, about 15 minutes. Squeeze out garlic pulp, and place in a medium bowl. Add vinegar, rosemary, mustard, salt, and lemon zest, and stir until smooth. Cover and chill until ready to use.
Remove chicken from brine. Discard brine. Pat chicken dry with paper towels. Spread garlic rub evenly over chicken breasts and legs. Place chicken on a wire rack set inside a rimmed baking sheet, and chill, uncovered, at least 4 hours and up to 12 hours. Remove from refrigerator; let stand at room temperature 1 hour.
Preheat oven to 375°F. Tie legs of chicken with kitchen twine, and tuck wing tips underneath. Roast chicken until a thermometer inserted in thigh registers 165°F, about 1 hour and 30 minutes, loosely tenting chicken with foil halfway through roasting.
Transfer chicken to a cutting board with a juice groove or to a flat cutting board set inside a rimmed baking sheet to catch juices. Let stand 10 minutes. Carve chicken, and transfer to a platter.
Make the board dressing: Scrape carving juices and pan drippings into a measuring cup, and add enough olive oil to equal 1/3 cup. Add shallot, parsley, tarragon, lemon zest and juice, and pepper; stir to combine. Serve dressing with chicken.
Try this method with turkey. Sub the herbs for dried chiles or other spices.