To spatchcock the chicken, place the chicken with the back facing you (the drumsticks should be facing up). Use sharp kitchen shears and cut 3/4" to each side of the backbone (center of the chicken) all the way up. Remove the backbone and save for stock. Lay the chicken flat, skin side up and press down on the breast with your palms to flatten the chicken.
Place the garlic, ginger, miso, cooking oil and mirin into a resealable bag. Add in the chicken and massage to coat the chicken all over with the miso marinade. Refrigerate for 30 minutes or overnight.
Heat the oven to 425F. Place the chicken, skin side up in roasting pan. Lay the chicken flat and arrange the legs so that the thighs are also facing up (all skin should be facing up - see photos) Roast for 20 minutes. Cover chicken loosely with tin foil. Continue roasting for additional 25-30 minutes or until interior of breast registers 165F. Remove from oven, let rest for 15 minutes.