Preheat your oven to 400°F (200°C) and line a large rimmed sheet pan with parchment paper for easy cleanup.
Slice the chicken breast into even 1-inch strips to ensure they cook quickly and stay juicy.
Chop the broccoli into bite-sized florets, slice the red bell pepper into strips, and wedge the red onion.
In a large mixing bowl, whisk together the olive oil, harissa paste, minced garlic, sea salt, and black pepper.
Add the chicken and vegetables to the bowl, tossing thoroughly until every piece is coated in the vibrant red spice paste.
Spread the mixture in a single layer on the prepared sheet pan, ensuring nothing is overcrowded so the vegetables roast rather than steam.
Roast for 20 to 25 minutes, or until the chicken is cooked through and the vegetables have developed slightly charred, caramelized edges.
Remove from the oven and immediately drizzle with fresh lemon juice to brighten the deep flavors.
Garnish with chopped fresh parsley and serve hot for a clean, high-protein meal with a serious kick.