Place the bird on its breast and locate the spine. On either side of the backbone cut down the length of the spine using kitchen shears. Snip through the ribs about 1/2 inch to an inch away from the spine. Remove the back bone.
Flip the bird over so that the breast is facing up. Press down hard on the breast to break the bones on the rib cage so the bird will lie flatter.
Dice vegetables and scatter on a sheet pan. Add herbs.
Place the bird on the vegetables and arrange the legs so that they lie as flat as possible on the pan without breaking the joints in the leg or tearing the skin. Tuck the wings under the rib cage to protect them as they cook.
Roast for 20 minutes at 450 degrees F. Reduce temperature to 400 degrees F and continue roasting until the deepest part of the breast reaches 159 degrees F. A 20 pound bird takes about 90 minutes.