Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Cut the chicken breast into 1-inch cubes and place them in a large mixing bowl.
Chop the red bell pepper, zucchini, and red onion into similar bite-sized pieces and add them to the bowl with the chicken.
In a small ramekin, whisk together the harissa paste, extra virgin olive oil, lemon juice, and minced garlic.
Pour the harissa dressing over the chicken and vegetables, tossing thoroughly until every piece is brightly coated.
Spread the mixture in a single layer onto the prepared baking sheet, ensuring the pieces are not overcrowded.
Sprinkle the sea salt and black pepper evenly over the top.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender and slightly charred.
Serve immediately, drizzling any remaining juices from the pan over the chicken.