Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Cut the chicken breast into 1-inch cubes and chop the zucchini, bell pepper, and red onion into similar bite-sized pieces.
In a large mixing bowl, whisk together the harissa paste, olive oil, minced garlic, sea salt, and black pepper to create the marinade.
Add the chicken and all the chopped vegetables to the bowl, tossing thoroughly until every piece is evenly coated in the spicy harissa mixture.
Spread the chicken and vegetable mixture in a single layer across the prepared baking sheet, ensuring they are not overcrowded to allow for proper roasting.
Place in the oven and roast for 20 to 25 minutes, tossing halfway through, until the chicken is cooked through and the vegetables have developed a slight char.
Remove from the oven, drizzle with fresh lemon juice for a bright finish, and serve immediately.