Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Cut the chicken breast into 1-inch bite-sized pieces and place them in a large mixing bowl.
Add the cauliflower florets, sliced red bell pepper, and sliced red onion to the bowl with the chicken.
In a small ramekin, whisk together the harissa paste and extra virgin olive oil until smooth.
Pour the harissa mixture over the chicken and vegetables, then sprinkle with sea salt and black pepper.
Toss everything thoroughly with your hands or a spatula to ensure an even, spicy coating on every piece.
Spread the mixture in a single layer on the prepared baking sheet, ensuring nothing is overcrowded so it roasts rather than steams.
Bake for 20 to 25 minutes, or until the chicken is cooked through and the vegetables have developed dark, caramelized edges.
Remove from the oven and immediately drizzle with fresh lemon juice to brighten the flavors.
Garnish with chopped fresh parsley and serve warm.