3tablespoonsGrand Marnier or other orange liquoror orange juice
Preheat the oven to 425°F.
Place the bird on a cutting board breast side up. Using a very sharp knife, cut between the thighs and the main body of the bird so that the thighs can open up and lay fairly flat. Place the chicken in a cast iron pan. Squeeze the orange segments over the bird and season with salt and pepper all over. Place the squeezed orange segments and the onion chunks into the cavity of the bird. Dot the chicken with two tablespoons of the cut up butter and sprinkle with the rosemary.
Roast for 45 minutes. While the chicken is roasting, in a small saucepan heat the water, mustard, honey, Grand Marnier or orange juice, and remaining tablespoon of butter, stirring until smooth. After 25 minutes, brush the chicken all over with the glaze. Roast for another 20 minutes, basting with the liquid at the bottom of the pan occasionally, until the juices run clear and an internal thermometer stuck into the thickest part of the thigh registers 160°F. If the top is browning too quickly, ten the chicken with foil and continue cooking.
Let sit for about 10 minutes before carving. Serve with the juices from the roasting pan.