Preheat oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Cut the chicken breast into 1-inch cubes and place them in a mixing bowl.
Chop the zucchini, bell pepper, and red onion into bite-sized pieces and add them to the bowl with the chicken.
In a small jar or bowl, whisk together the harissa paste, olive oil, lemon juice, sea salt, black pepper, and garlic powder.
Pour the harissa marinade over the chicken and vegetables, tossing thoroughly until everything is evenly coated in the spicy sauce.
Spread the mixture in a single layer on the prepared baking sheet, ensuring there is space between pieces for even roasting.
Roast for 18-22 minutes, or until the chicken is cooked through and the vegetables are tender and slightly charred at the edges.
Remove from the oven and let rest for 2 minutes before serving hot.