Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Cut the chicken breast into 1-inch cubes and chop the red bell pepper, zucchini, and red onion into similarly sized pieces to ensure even cooking.
In a large mixing bowl, whisk together the harissa paste, extra virgin olive oil, lemon juice, sea salt, black pepper, and garlic powder until a smooth marinade forms.
Add the chicken cubes and chopped vegetables to the bowl, tossing thoroughly until every piece is generously coated in the spicy harissa mixture.
Spread the chicken and vegetable mixture in a single layer across the prepared baking sheet, leaving space between pieces so they roast rather than steam.
Place the baking sheet in the center of the oven and roast for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender with caramelized edges.
Remove from the oven and let the dish rest for 2 minutes before serving warm.