Place the whole chicken breast-side down with the wings facing towards you. Remove the spine by using kitchen shears to cut out the backbone along both sides of the spine. Open the rib cage, spread the chicken out and use a knife to score down the sternum. This will help pop out the breast bone and easily flatten chicken.
Flip the chicken over so that the breast is facing up. Use the palm of your hand and push firmly over the breast bone, until you hear the wishbone cracking. This helps the chicken to lay completely flat.
Brine and Marinate
Mix together salt, sugar, orange juice and water in a large pot. Immerse the chicken completely in this, sprinkle over some black pepper and add a few sprigs of rosemary. Refrigerate for 2-3 hours.
When ready to cook, preheat the oven at 218 C/425 F for at least 15-20 minutes.
In a bowl mix together butter, garlic, orange zest, chopped rosemary, salt and pepper. Remove the chicken from the brine and pat dry with a kitchen towel. Now, use your thumb between the skin and the flesh over the breast and the legs to make space to stuff the butter in. Spread half the butter under the skin and the remaining all over the chicken.
In a 12 inch cast iron skillet, or on a baking sheet, layer the orange slices at the bottom. Add onions, garlic and the white wine to the pan and then place the chicken breast side up. Roast for 40 mins in the oven, temperature of the thigh should be at 80C/ 175F.
Allow the chicken to rest for 15 minutes before serving. Serve over mashed potatoes, with the juices from the pan.