Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Cut the chicken breast into 1-inch pieces and chop the bell pepper, zucchini, and red onion into similar-sized chunks.
In a large mixing bowl, whisk together the harissa paste, olive oil, lemon juice, garlic powder, sea salt, and black pepper until well combined.
Add the chicken and chopped vegetables to the bowl, tossing thoroughly to ensure every piece is evenly coated in the spicy marinade.
Spread the chicken and vegetable mixture in a single layer across the prepared baking sheet, avoiding overcrowding to allow for proper roasting.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender with slightly charred edges.
Remove from the oven and let rest for 2 minutes before serving to allow the juices to redistribute.