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 extra virgin olive oil, harissa paste, lemon juice, and minced garlic until well combined.
Cut the chicken breast into 1-inch cubes and chop the cauliflower, red bell pepper, and red onion into uniform, bite-sized pieces.
Place the chicken and vegetables into a large bowl, pour the harissa mixture over them, and toss thoroughly to ensure every piece is coated.
Spread the chicken and vegetable mixture in a single layer onto the prepared baking sheet, ensuring they are not crowded to allow for proper roasting.
Season the tray with sea salt and black pepper, then roast in the oven for 20 to 25 minutes until the chicken is cooked through and the vegetables are tender with charred edges.
Remove from the oven and garnish with freshly chopped cilantro before serving warm.