Prepare the marinade in a small bowl - mix together the chopped garlic, pimentón, dried oregano and olive oil.
Line a baking tray with greaseproof baking paper. Now it’s time to spatchcock that chicken: using sharp kitchen scissors, cut the chicken either side of the backbone. Remove and discard the backbone. Open the chicken out, turn it over and flatten it by pressing down with the heel of your hand along the breastbone. Place the now spatchcocked chicken on the prepared baking tray. Spread the marinade over the chicken, cover and leave to marinate in the fridge overnight or for at least 6 to 8 hours.
Preheat the oven to 200C/180C fan.
If you’re including the potatoes, peppers and red onion, arrange these around the chicken on the tray, drizzle with olive oil and season with salt and freshly ground pepper.
Also now season the chicken with salt and freshly ground black pepper. Roast for 60 to 70 minutes, occasionally (at least 3 times) brushing the pan juices back over the chicken and the vegetables. Check that the temperature of the breasts are 74C – I use my Thermapen to do this.
Remove the chicken from the oven, transfer the potatoes to a serving dish, and keep warm. Cover the chicken with tin foil and let it rest for 5 minutes.
Serve the chicken with the optional potatoes and vegetables, or some chunky oven chips and salad.