Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
In a large mixing bowl, whisk together the harissa paste, olive oil, lemon juice, sea salt, and black pepper until well combined.
Cut the chicken breast into 1-inch bite-sized cubes and add them to the bowl with the marinade.
Chop the red bell pepper and zucchini into similar-sized pieces, and slice the red onion into thick wedges, then add them to the bowl.
Toss the chicken and vegetables thoroughly until every piece is generously coated in the harissa mixture.
Spread the mixture in a single, even layer on the prepared baking sheet, ensuring the pieces aren't overcrowded.
Roast in the center of the oven for 18 to 22 minutes, or until the chicken is cooked through and the vegetables are tender with slightly charred edges.
Remove from the oven and garnish with freshly chopped parsley for a bright finish before serving.