Preheat your oven to 425°F (220°C) and line a large baking sheet with parchment paper.
Chop the cauliflower into small florets, slice the red bell pepper into strips, and roughly dice the red onion.
In a large mixing bowl, whisk together the harissa paste, extra virgin olive oil, minced garlic, lemon juice, sea salt, and black pepper.
Add the chicken breast and the prepared vegetables to the bowl, tossing thoroughly to ensure every piece is coated in the spicy marinade.
Spread the chicken and vegetables in a single, even layer on the prepared baking sheet.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables have developed charred, caramelized edges.
Remove from the oven and let the chicken rest for 5 minutes before slicing.
Garnish the entire dish with chopped fresh parsley and serve immediately.