Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Chop the red bell pepper, zucchini, and red onion into uniform 1-inch pieces to ensure even roasting.
In a small mixing bowl, whisk together the olive oil, harissa paste, lemon juice, garlic powder, sea salt, and black pepper until well combined.
Place the chicken breast and the chopped vegetables onto the prepared baking sheet in a single layer.
Drizzle the harissa marinade over the chicken and vegetables, using your hands or a brush to coat every piece thoroughly.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender and slightly caramelized.
Remove from the oven and let the chicken rest for 5 minutes before slicing it into strips and serving with the roasted vegetables.