Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
In a small mixing bowl, whisk together the harissa paste, half of the olive oil, lemon juice, garlic powder, sea salt, and black pepper until a smooth paste forms.
Slice the chicken breast into even 1-inch thick strips and toss them in the harissa mixture until every piece is thoroughly coated.
Place the cauliflower florets, bell pepper strips, and red onion wedges on the prepared baking sheet, drizzle with the remaining olive oil, and toss to coat.
Spread the vegetables toward the edges of the sheet and place the marinated chicken strips in the center, ensuring nothing is overcrowded.
Roast in the center of 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.