Preheat your oven to 425°F and line a large rimmed baking sheet with parchment paper.
Cut the cauliflower into bite-sized florets, slice the red bell pepper into thin strips, and roughly dice the red onion.
In a large mixing bowl, whisk together the harissa paste, extra virgin olive oil, lemon juice, sea salt, and black pepper until well combined.
Slice the chicken breast into 1-inch cubes and add them to the bowl, tossing until every piece is thoroughly coated in the spicy marinade.
Add the prepared cauliflower, bell peppers, and onions to the bowl and toss again to lightly coat the vegetables with the remaining marinade.
Spread the chicken and vegetable mixture in a single even layer across the prepared baking sheet.
Roast for 20 to 22 minutes, or until the chicken is cooked through and the vegetables have developed charred, caramelized edges.
Garnish the dish with freshly chopped cilantro and an extra squeeze of lemon if desired before serving warm.