Preheat your oven to 400°F and line a large baking sheet with parchment paper.
In a small bowl, whisk together the harissa paste, half of the olive oil, minced garlic, and lemon juice to create a marinade.
Place the chicken breast on one side of the baking sheet and coat it thoroughly with half of the harissa mixture.
In a separate bowl, toss the broccoli florets, sliced bell pepper, and red onion with the remaining olive oil, sea salt, and black pepper.
Spread the vegetables in a single layer on the other side of the baking sheet, ensuring they are not overcrowded.
Roast for 20 to 25 minutes until the chicken reaches an internal temperature of 165°F and the vegetables are tender with slightly charred edges.
Let the chicken rest for 5 minutes before slicing and drizzling with the remaining harissa sauce for an extra kick of flavor.