Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
In a small mixing bowl, whisk together the harissa paste, half of the olive oil, and the lemon juice until well combined.
Place the chicken breast on one side of the prepared baking sheet and coat it thoroughly on all sides with the harissa mixture.
In a separate bowl, toss the cauliflower florets, sliced red bell pepper, and red onion with the remaining olive oil, sea salt, black pepper, and ground cumin.
Spread the seasoned vegetables in a single layer on the other side of the baking sheet, ensuring they aren't overcrowded so they can char properly.
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 are tender with browned edges.
Remove from the oven and let the chicken rest for 5 minutes before slicing it into strips.
Plate the chicken alongside the roasted vegetables and garnish with a sprinkle of fresh chopped parsley for a bright finish.