Spatchcocking is more simple than it sounds. Using a sharp pair of kitchen shears, remove the backbone of the bird and flatten it before cooking for extra crispy skin in less than the usual cooking time.
Preheat oven to 450°. On work surface, place chicken breast side down. Starting at thigh end, with kitchen scissors, cut along 1 side of backbone; repeat on other side of backbone to remove bone. Turn chicken over; press down firmly on breastbone to flatten chicken. Pat chicken dry with paper towels.
In 3-quart baking dish, toss garlic, shallots, celery, lemon and sherry. Place chicken breast side up on top of vegetables; rub with oil, rosemary, sage, thyme, 3/4 teaspoon salt and 1/2 teaspoon pepper. Tuck wings behind breasts to prevent burning.
Roast chicken and vegetables 45 minutes; increase temperature to 500°. Roast 10 minutes or until internal temperature of chicken reaches 165°, rotating once. Transfer chicken to cutting board; loosely cover with aluminum foil and let stand 10 minutes.
With tongs, transfer 20 garlic cloves to blender; strain drippings from baking dish into blender. Blend on high 1 minute; strain through a fine-mesh strainer into small saucepot. Add remaining 1/4 teaspoon each salt and pepper; cook over medium-high heat 8 minutes or until reduced by half.