Heat oven to 400 degrees F and arrange a rack in the middle. Pat chicken dry with paper towels. Remove necks and any innards from the cavities and discard.
To spatchcock, use a sharp pair of poultry shears or kitchen scissors to cut along the backbone; remove and discard. Place chicken, breast-side up, and press down to flatten it to it is lying completely flat.
Combine remaining ingredients in a small bowl and mix until evenly combined. Season and adjust flavor as desired. Loosen the skin from the breast and legs of the chicken and, using your clean hands, spread 1/2 of the mixture underneath. Rub the remaining mixture all over the outside of the chicken. Set aside while oven heats up, at least 20 minutes.
Lay chicken in a roasting pan or baking dish fitted with a rack and roast until cooked an instant read thermometer inserted into the thigh registers 165 degrees F, about 50 to 60 minutes; let sit at least 10 minutes before carving. Serve with pan juices.