Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Cut the chicken breast into one-inch cubes and chop the zucchini, bell pepper, and red onion into similar bite-sized pieces.
In a large mixing bowl, whisk together the harissa paste, olive oil, lemon juice, minced garlic, sea salt, and black pepper until smooth.
Add the chicken and vegetables to the bowl, tossing thoroughly to ensure every piece is coated in the spicy marinade.
Spread the mixture in a single, even layer onto the prepared baking sheet to ensure even roasting.
Place in the oven and roast for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender with charred edges.
Remove from the oven and let rest for two minutes before serving to allow the juices to redistribute.