Preheat oven to 200 C / Gas 6. Grease the top and bottom of a roasting tin with cooking spray.
Layer sweet potatoes, potatoes and onion slices in the bottom of the roasting tin.
Place chicken, breast side down, on a clean work surface. Cut along both sides of the backbone with kitchen shears from top to bottom; remove backbone. Flip chicken and splay it open. Press firmly onto breasts with your hands to break the breast-bone.
Brush olive oil over chicken. Season with salt and pepper. Place on top of the vegetables in the roasting tin.
Roast in the preheated oven until juices run clear and chicken is cooked through, about 1 hour.
Let cool before slicing, about 10 minutes. Serve with potato and onion mixture.