1. Remove and discard giblets and neck from chicken. Rinse chicken, and pat dry. Place chicken, breast side down, on a cutting board. Cut chicken, using kitchen shears, along both sides of backbone, separating backbone from chicken; discard backbone. Open chicken as you would a book. Turn chicken, breast side up, and press firmly against breastbone with the heel of your hand until bone cracks. Tuck wing tips under. Place chicken in a baking dish or pan.
2. Pulse garlic and next 8 ingredients in a food processor until mixture forms a thick paste. Reserve half of paste. Rub remaining half of paste on both sides of chicken. Cover chicken with plastic wrap, and chill 1 hour.
3. Heat one side of grill to 300º to 350° (medium) heat; leave other side unlit. Place chicken, breast side down, over lit side of grill; top with a piece of aluminum foil. Place a cast-iron skillet on foil-topped chicken to flatten. Grill, covered with grill lid, 10 to 15 minutes or until chicken is browned. Remove skillet and foil. Turn chicken over, and transfer to unlit side of grill. Grill, covered with grill lid, 45 minutes or until a meat thermometer inserted into thickest portion of breast registers 165°. Remove chicken from grill, and let stand 5 minutes. Brush with reserved paste.