Place the chicken breast side down, use a kitchen shears/scissor and cut the backbone off.
Then turn upside down (the breast side facing you) then use the hill of your hand to press the sternum bone this helps to flatten the rib cage and chicken breasts.
Roast in the preheated oven until golden brown about 35 minutes or the thickest part register 165-170 degrees F in the thermometer or until runs clear. The time may differ so have that at the back of your mind. After 30 minutes start to check the chicken.
When it is done, allow the chicken to rest for 10 minutes before carving. Serve and enjoy.