Preheat your oven to 425°F (220°C) and line a large rimmed baking sheet with parchment paper.
In a small mixing bowl, whisk together the harissa paste, extra virgin olive oil, lemon juice, minced garlic, ground cumin, sea salt, and black pepper to create the marinade.
Slice the chicken breast into bite-sized 1-inch chunks and chop the red bell pepper and red onion into similar sized pieces.
Place the chicken chunks, broccoli florets, bell pepper, and onion onto the prepared baking sheet.
Drizzle the harissa marinade over the chicken and vegetables, using your hands or tongs to toss everything until every piece is thoroughly and evenly coated.
Spread the mixture out into a single layer, ensuring the chicken and vegetables are not overcrowded to allow for proper browning.
Roast in the center of the oven for 18 to 22 minutes, or until the chicken is cooked through and the vegetables have developed slightly charred, caramelized edges.
Remove from the oven and let rest for 2 minutes before serving warm.