Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Cut the chicken breast into bite-sized 1-inch cubes and slice the bell pepper, zucchini, and red onion into uniform pieces.
In a large mixing bowl, whisk together the harissa paste, avocado oil, lemon juice, garlic powder, sea salt, and black pepper to create the marinade.
Add the chicken cubes and all the sliced vegetables to the bowl, tossing thoroughly until every piece is well-coated in the spicy harissa mixture.
Spread the chicken and vegetables in a single, even layer on the prepared baking sheet to ensure even roasting.
Roast in the center of the oven for 20 to 22 minutes, or until the chicken is cooked through and the vegetables have developed deep, caramelized edges.
Remove from the oven and let rest for 2 minutes before serving to allow the juices to redistribute.