Preheat the oven to 400ºF (205ºC). Line a baking sheet with parchment paper.
Cook the barley according to the package directions. Drain excess water and season with 1/3 of the harissa. Season with salt and pepper.
Meanwhile, toss the chicken, courgette, bell pepper, and onion with the remaining harissa, oil, salt, and pepper and arrange on the prepared baking sheet. Spread evenly and cook in the oven for 20 to 25 minutes, or until the vegetables are tender and the chicken is cooked through.
Add the cooked barley, roasted vegetables, chicken, parsley, and lemon juice to a serving dish and toss to coat. Divide evenly between plates and enjoy!