Preheat your oven to 450°F (232°C).

Zest both oranges and juice one, yielding about ½ cup (120 ml) of juice. Slice the other orange into wedges and set these aside.

Combine the orange zest with the softened butter and stir. Set this mixture aside.

Take 2 tablespoons (28 g) of the orange zest butter and 1 teaspoon of salt, and coat the outside and underneath the skin of the chicken evenly.

While the skin is pulled back, place 4 sprigs of rosemary on top of the chicken meat. Cover the rosemary with the chicken skin.

In a large cast-iron skillet, add the olive oil and onion. Warm over medium-high heat and cook the onion for about 3 minutes until slightly soft.

Push the onions to the edge of the skillet and add the chicken, breast-side down. Cook for 2½ minutes, then flip the chicken.

Add the orange wedges and the remaining 3 sprigs of rosemary to the skillet. Cook for another 2½ minutes.

Transfer the skillet to the oven. Roast for around 30 minutes or until the chicken's internal temperature reaches 160°F to 165°F (71°C to 74°C).

Take the skillet out of the oven, move the chicken to a cutting board, cover with foil, and let it rest for 10 minutes.

Return the skillet to the cooktop and warm over medium-high heat. Add the chicken stock, orange juice, and ¼ teaspoon of salt.

Bring to a boil and let it boil for 2 to 3 minutes, stirring occasionally.

Remove from heat, slice the chicken, and serve with the pan sauce.