Preheat the oven to 400°F. Spray a large rimmed sheet pan or roasting pan with cooking spray, then arrange lemon slices in center of pan to form a rack to hold the chicken. Set aside.
Using poultry shears, cut along each side of the chicken backbone and remove it. Flip the chicken breast side up and press on the breastbone to flatten the chicken. (You can also use a skillet or a sheet pan to press and flatten.)
In a small bowl mix together olive oil, butter, garlic and onion powder. Rub the chicken all over with the olive oil-butter mixture then season with oregano, thyme, and rosemary. Generously season on all sides with salt and lemon pepper.
Transfer the chicken to a large rimmed baking sheet or a roasting pan over the lemon slices, skin side up.
Roast for 40-45 minutes or until the skin is browned and the internal temperature of the inner thigh reaches 160-165°F.
Allow chicken to rest for 10 minutes, then carve and serve garnished with lemon and rosemary, if desired.