Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Slice the chicken breast into 1-inch bite-sized cubes and set aside.
In a large mixing bowl, whisk together the harissa paste, olive oil, lemon juice, sea salt, black pepper, and garlic powder until smooth.
Chop the red bell pepper and red onion into 1-inch pieces, keeping them similar in size to the broccoli florets.
Add the chicken cubes and all the vegetables into the bowl with the harissa marinade, tossing thoroughly until every piece is well-coated.
Spread the mixture in a single layer onto the prepared baking sheet, ensuring enough space for the air to circulate.
Roast for 18 to 22 minutes, or until the chicken is cooked through and the vegetables have developed a light char on the edges.