Preheat the oven to 190°C.
Cut the onion and one orange into thin rings.
Squeeze the juice out of the other orange, then cut it into 4 slices.
Mix the butter or spread with the mixed spice, salt and pepper. Add 1 Tbsp orange juice.
Butterfly/spatchcock the chicken. Place it on a chopping board, breast-side down, then remove its backbone, using sharp scissors. Turn over the chicken and flatten it by pushing down the breastbone.
Rub the chicken all over, then under the skin with the spiced butter. Carefully push some orange slices under the skin (over the thighs and over the breast).
Put the onion rings, rosemary sprig and the orange ring on a rack, then place the chicken (skin-side facing upwards) on top. Put the rack over a roasting tray. Season the chicken with salt and pepper, then put the dish in the oven.
Bake for about 45 minutes until golden, basting the chicken with the jus and the remaining orange juice every 10 minutes. If it’s getting brown too soon, cover the chicken with foil.
Test the chicken at the thighs with a skewer. If clear jus is coming out, it’s cooked through. If it's rose, roast for another 5 minutes and test again.
When the chicken is cooked, remove it to a plate, together with the orange and onion pieces, then cover and rest for 10 minutes.
While the chicken is resting, make a gravy from the cooking jus and any remaining orange juice.
Cut the chicken into four portions and serve with the gravy and side dishes of your choice.