Preheat your oven to 425°F (220°C).
If not already spatchcocked, use a sharp knife or kitchen shears to cut along both sides of the chicken's backbone and remove it. Flip the chicken over and press down firmly on the breastbone to flatten.
Place the potato wedges, red onion slices, and halved mini bell peppers on a large baking tray. Drizzle with 1 tablespoon of olive oil and season with salt and pepper. Toss to coat evenly.
Place the spatchcocked chicken on top of the vegetables. Rub the chicken all over with the harissa paste, making sure to get under the skin where possible. Drizzle with the remaining 1 tablespoon of olive oil and season with salt and pepper.
Arrange the lemon slices around the chicken and vegetables on the baking tray.
Place the tray in the preheated oven and bake for 45-50 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the juices run clear.
Remove the chicken from the oven and let it rest for a few minutes. Garnish with pomegranate arils and chopped cilantro, if using.
