Mix all the spices together.: The aroma will bloom immediately, releasing warm, sweet, and earthy notes that tell you the rub is ready. As you stir, inhale deeply to gauge balance, because the spice mix sets the dish's personality. Use a small bowl and whisk until the colors are evenly blended, and if you notice one spice dominating, adjust slightly. A common mistake is adding too much of a single spice, which can throw the blend off, so measure carefully.
Cut the chicken along the middle of the breast and open the chicken in a pan, then season well on all sides with the spice mixture.: When you butterfly the chicken , you create more surface area for the spices to cling to, resulting in a more flavorful crust. As you press the spice rub into the skin, you should feel some resistance and see the blend adhere; that is a good sign. If the spices seem to slide off, pat the skin dry first to help adhesion. Avoid forcing the bird into a pan that is too small, because overcrowding prevents even seasoning and browning.
Cover the pan with foil and refrigerate for 1-2 hours or even longer to allow the chicken to absorb well the flavor from the spice mixture.: Cooling the seasoned chicken lets the flavors penetrate the meat gradually, deepening the final taste. As it chills, the spices mingle with the meat juices, so you will notice a subtle, mellowing scent when you uncover it. If you press on the breast and it feels firmer after marinating, that is normal as the salt begins to work. Do not skip this step if you want more pronounced flavor, and avoid marinating at room temperature which risks bacteria growth.
Clean and cut the vegetables into large pieces and cook them with the chicken.: Large chunks of carrots , parsnips , celery , and beetroot roast evenly alongside the chicken without disintegrating. As you cut, aim for uniform sizes so each piece reaches the same level of tenderness. Toss them lightly in a little of the reserved spice mix and olive oil , so they pick up flavor and roast into glossy, caramelized edges. A frequent slip is chopping pieces unevenly, which leads to some items overcooking while others remain underdone.
Place the tray with the chicken and vegetables in the oven heated to 180 °C (356 °F), drizzle with olive oil and cook for about 1-1.5 hours.: As the oven heats, you will soon notice the kitchen filling with an intense, warm aroma, and the chicken skin will begin to bronzed and tighten. The vegetables will sizzle in the pan juices and slowly caramelize, giving sweet, nutty edges. Aim for a golden brown skin and tender vegetables that yield to a fork, while juices run clear from the thickest part of the chicken . If the skin begins to darken too quickly, tent with foil to prevent burning while allowing the interior to finish cooking. Rushing this step by using too high a temperature risks drying the meat and charring the vegetables.