Preheat your oven to 400°F (200°C) 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 the glaze.
Chop the broccoli, red bell pepper, and red onion into uniform, bite-sized pieces and place them on the baking sheet.
Toss the vegetables with the remaining olive oil, sea salt, and black pepper until evenly coated.
Place the chicken breast on the baking sheet, nestling it among the vegetables, and brush the harissa glaze over the chicken until fully covered.
Roast for 20-25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender and slightly charred.
Remove from the oven and let the chicken rest for 5 minutes before slicing to keep it juicy, then serve with the roasted vegetables.