Preheat your oven to 425°F and line a large rimmed baking sheet with unbleached parchment paper.
In a small glass bowl, whisk together the harissa paste, extra virgin olive oil, and fresh lemon juice until emulsified.
Prepare the vegetables by cutting the cauliflower into bite-sized florets, slicing the bell pepper into strips, and cutting the onion into thin wedges.
Place the chicken breast and all the chopped vegetables onto the prepared baking sheet.
Pour the harissa mixture over the chicken and vegetables, then sprinkle with sea salt and black pepper.
Use your hands or tongs to toss everything together, ensuring every piece is coated in that spicy, aromatic marinade.
Roast for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the cauliflower edges are beautifully charred.
Remove from the oven and let the chicken rest for 5 minutes before slicing into thick, juicy strips.
Garnish with chopped fresh parsley and serve warm.