Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
In a small mixing bowl, whisk together the spicy harissa paste, extra virgin olive oil, lemon juice, minced garlic, sea salt, and black pepper until well combined.
Place the chicken breast, sliced zucchini, chopped red bell pepper, and sliced red onion into a large bowl and pour the harissa marinade over them, tossing thoroughly to coat every piece.
Spread the marinated chicken and vegetables in a single layer on the prepared baking sheet, ensuring there is enough space between pieces to allow for roasting rather than steaming.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender with charred, caramelized edges.
Remove from the oven and let the chicken rest for 5 minutes before slicing it into strips; garnish the entire dish with fresh chopped parsley and serve immediately.