If the chicken is not already split/butterflied, place the chicken breast-side down and use kitchen shears to cut along both sides of the backbone; remove backbone and press firmly on the breastbone to flatten (spatchcock). Pat the chicken dry inside and out with paper towels. This helps the marinade adhere and promotes browning.

Make the herb marinade

In a medium bowl combine 4 tbs olive oil, chopped rosemary, thyme, parsley, minced garlic, lemon zest and juice from 1 lemon, 1 tsp Dijon mustard, 1.5 tsp kosher salt, 1 tsp ground black pepper and 1/4 tsp crushed red pepper flakes. Whisk until emulsified.

Marinate the chicken

Place the split chicken in a shallow dish or large resealable bag. Pour the herb marinade over the chicken and rub thoroughly so marinade reaches the skin and the cavity. If possible, loosen the skin over the breasts and thighs and work a little marinade underneath. Cover and refrigerate for at least 2 hours, preferably overnight (up to 24 hours) for best flavor. Bring the chicken to near room temperature (about 30 minutes) before roasting.

Roasting & Finishing

Prepare vegetables and oven

Preheat oven to 220°C (425°F). Toss the quartered onion, halved carrots and potatoes with 1 tbs olive oil (from extra or use the 2 tbs butter later), a pinch of salt and pepper. Spread the vegetables in an even layer on a large rimmed baking sheet or roasting pan, leaving a space in the center for the chicken.

Arrange chicken and baste

Place the marinated chicken skin-side up on the vegetables in the roasting pan, breast in the center with legs splayed. Tuck optional lemon slices around and under the chicken. Pour 120 ml chicken or vegetable stock into the pan (not over the skin) to create steam and pan juices. Brush the skin lightly with the 2 tbs melted butter or remaining 1 tbs olive oil for extra browning.

Roast the chicken

Roast at 220°C (425°F) for 20 minutes to develop a golden crust, then reduce heat to 190°C (375°F) and continue roasting for an additional 25–35 minutes, or until an instant-read thermometer inserted into the thickest part of the thigh away from bone reads 74°C (165°F). Total roasting time should be about 45–55 minutes depending on size. Spoon pan juices over the chicken once or twice during cooking.

Rest and finish

Remove the pan from the oven and transfer the chicken to a cutting board. Tent loosely with foil and let rest for 10–15 minutes so juices redistribute. While resting, if the vegetables need extra browning, return them to the oven for 5–8 minutes.

Carve and serve

Carve the chicken into portions (breasts, thighs, legs, wings). Spoon the pan-roasted vegetables and any strained pan juices over the meat. Sprinkle with chopped fresh parsley and an extra squeeze of lemon if desired. Serve hot. Leftovers: cooled and refrigerated, the chicken keeps well and can be reheated gently in a 180°C (350°F) oven covered for 15–20 minutes or warmed in slices.