Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Cut the chicken breast into 1-inch cubes and place them in a large mixing bowl.
Prepare the vegetables by breaking the cauliflower into small florets, slicing the bell pepper into strips, and cutting the red onion into thick wedges.
In a small bowl, whisk together the harissa paste, olive oil, lemon juice, sea salt, and black pepper to create the marinade.
Add all the vegetables to the bowl with the chicken and pour the harissa marinade over the top.
Toss everything together with your hands or a spatula until every piece is thoroughly and evenly coated with the spicy sauce.
Spread the mixture out in a single layer on the prepared sheet pan, ensuring there is space between pieces so they roast rather than steam.
Roast for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender with dark, caramelized edges.
Remove from the oven and garnish with freshly chopped cilantro before serving immediately.