Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Slice the chicken breast into 1-inch strips and place them in a large mixing bowl.
Chop the red bell pepper, zucchini, and red onion into bite-sized pieces and add them to the bowl with the chicken.
In a small jar, whisk together the harissa paste, olive oil, minced garlic, lemon juice, sea salt, and black pepper.
Pour the harissa marinade over the chicken and vegetables, tossing thoroughly to ensure everything is evenly coated.
Spread the mixture in a single layer on the prepared baking sheet, ensuring the pieces are not overcrowded.
Roast for 20-25 minutes, or until the chicken is cooked through and the vegetables are tender and slightly charred.
Garnish with freshly chopped cilantro before serving hot.