Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
In a small mixing bowl, whisk together the harissa paste, half of the olive oil, lemon juice, minced garlic, sea salt, and black pepper until well combined.
Trim the Brussels sprouts and cut them in half, and slice the red bell pepper into thin strips.
Place the chicken breast on one side of the baking sheet and arrange the vegetables on the other side.
Brush the harissa mixture generously over all sides of the chicken breast.
Drizzle the remaining olive oil over the Brussels sprouts and peppers, tossing them lightly to coat.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender with caramelized edges.
Remove from the oven and let the chicken rest for 5 minutes before slicing and serving alongside the roasted vegetables.