Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
In a small bowl, whisk together the harissa paste, olive oil, lemon juice, sea salt, and black pepper until well combined.
Place the chicken breast, cauliflower florets, sliced red bell pepper, and red onion onto the prepared sheet pan.
Pour the harissa mixture over the chicken and vegetables, using your hands or tongs to toss everything until thoroughly and evenly coated.
Arrange the ingredients in a single layer, ensuring the chicken has enough space to roast properly without steaming.
Roast in the oven 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, let the chicken rest for 5 minutes, then slice the chicken and garnish the entire dish with fresh cilantro.