Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Slice the chicken breast into 1-inch strips and place them into a large mixing bowl.
Chop the red bell pepper, zucchini, and red onion into bite-sized chunks and add them to the bowl with the chicken.
In a small bowl, whisk together the harissa paste, olive oil, lemon juice, sea salt, black pepper, and garlic powder until smooth.
Pour the harissa marinade over the chicken and vegetables, tossing thoroughly with your hands or tongs to ensure every piece is well coated.
Spread the mixture in a single layer on the prepared baking sheet, making sure not to overcrowd the pan so the vegetables roast rather than steam.
Roast for 20-25 minutes, or until the chicken is cooked through and the vegetables have developed slightly charred, caramelized edges.
Remove from the oven and let the chicken rest for 5 minutes before serving to keep it juicy and flavorful.