Preheat the oven to 400 degrees F (200 degrees C). Grease both the bottom pan and the drip tray of a broiler pan with cooking spray.

Layer sweet potatoes, potatoes, and onion slices in the prepared bottom pan. Place the drip tray over top.

Place chicken onto a work surface with the breast facing down. Cut along both sides of the backbone with kitchen shears from top to bottom; remove the backbone. Flip chicken and splay it open. Press firmly onto the breasts with your hands to break the breastbone.

Brush olive oil over chicken. Season with salt and pepper. Place onto the drip tray of the broiler (the top).

Roast in the preheated oven until the juices run clear and an instant-read thermometer inserted into the thickest part of the thigh, near the bone, reads 165 degrees F (74 degrees C), about 1 hour.

Let cool before slicing, about 10 minutes. Serve with potato and onion mixture.