Preheat oven to 200°C.
To butterfly the chicken, place onto a chopping board breast side down. With good quality kitchen scissors cut either side of the backbone and remove it. Open the
chicken and flatten on the chopping board. Season the inside with salt and pepper.
In a baking tray combine the onions, sweet potatoes and rosemary and coat well with olive oil. Season with salt and pepper and pour over the wine.
Put chicken on top of the vegetables, flattened out, breast side up. Rub a little olive oil over the skin and season with salt and pepper.
Place the tray in the oven and roast for 45 mins. When the chicken is cooked the juices run clear when sticking a knife in the thigh.
Remove the chicken from the vegetables, cover with aluminum foil and leave to rest.
Baste the onions and sweet potato with the pan juices and return to the oven for a further 20 mins or until they are nicely caramelised.