Preheat oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Cut the chicken breast into 1-inch bite-sized cubes for even cooking.
Chop the broccoli into small florets, slice the bell pepper into thin strips, and cut the red onion into small wedges.
In a large mixing bowl, whisk together the harissa paste, olive oil, garlic powder, sea salt, and black pepper.
Add the chicken and vegetables to the bowl, tossing thoroughly until every piece is evenly coated in the spicy marinade.
Spread the mixture in a single layer on the prepared baking sheet, ensuring nothing is overcrowded to allow for proper roasting.
Bake for 18-22 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender and slightly charred.
Remove from the oven and drizzle with fresh lemon juice before serving to brighten the bold flavors.