Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
In a small bowl, whisk together the harissa paste, half of the olive oil, lemon juice, sea salt, and black pepper to create a thick marinade.
Cut the chicken breast into 1-inch cubes and toss them in the harissa mixture until every piece is thoroughly coated.
Chop the red bell pepper and red onion into bite-sized chunks, keeping them similar in size to the cauliflower florets.
Place the cauliflower, peppers, and onions on the baking sheet, drizzle with the remaining olive oil, and toss to coat.
Spread the vegetables out to the edges of the pan and place the marinated chicken pieces in the center, ensuring nothing is overcrowded.
Roast in the oven for 20-25 minutes, or until the chicken is cooked through and the vegetables have developed charred, caramelized edges.
Remove from the oven and garnish with freshly chopped parsley before serving hot.