Remove the tray from the oven and add the peeled garlic cloves and halved serrano chillies. Toss everything together and return to the oven for another 10 minutes. The garlic should be golden and fragrant, and the chillies should be softened.

Transfer all the roasted vegetables, including any juices from the tray, to a blender. Add the dried bird's eye chillies, white pepper, lemon zest and juice, and dried rosemary. Blend until smooth. Add the remaining oil and red wine vinegar, then blend again until you achieve a thick consistency. Set aside about two-thirds of the sauce for serving.

Place the chicken breast-side down on a sturdy chopping board with the legs facing you. Using heavy-duty scissors, carefully cut along both sides of the backbone. Take your time cutting through each rib bone as you go. Once both sides are cut, remove and discard the backbone (or save it for stock). Flip the chicken over so it's breast-side up, then press down firmly on the breastbone to flatten the chicken completely.

Place the spatchcocked chicken breast-side up on a baking tray. Pour about one-third of the peri peri sauce over the chicken, spreading it evenly with a brush or spoon. Sprinkle with the remaining ½ teaspoon salt and the black pepper. Place in the preheated oven at 180°C and roast for 45 minutes. After 45 minutes, remove the chicken from the oven and baste it with the pan juices. Spoon 2 more tablespoons of the reserved peri peri sauce over the chicken. Return to the oven for another 10-15 minutes until the chicken is cooked. Remove from the oven and let the chicken rest for 10 minutes before serving.