Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Cut the chicken breast into 1-inch cubes and slice the bell pepper, zucchini, and red onion into bite-sized pieces.
In a large mixing bowl, whisk together the olive oil, harissa paste, minced garlic, lemon juice, sea salt, and black pepper until well combined.
Add the chicken and chopped vegetables to the bowl, tossing thoroughly to ensure every piece is coated in the spicy marinade.
Spread the mixture in a single layer on the prepared baking sheet, ensuring enough space for the vegetables to roast rather than steam.
Roast for 20 to 25 minutes, or until the chicken is cooked through and the vegetables have developed slightly charred, golden edges.
Remove from the oven and let rest for 2 minutes before serving hot.