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 bite-sized cubes and place them in a large mixing bowl.
Chop the cauliflower into small bite-sized florets, slice the red bell pepper into thin strips, and cut the red onion into small wedges.
Add all the prepared vegetables into the bowl with the chicken cubes.
In a small bowl, whisk together the extra virgin olive oil, harissa paste, sea salt, and black pepper until well combined.
Pour the harissa marinade over the chicken and vegetables, using your hands or a spatula to toss until every piece is thoroughly and evenly coated.
Spread the mixture in a single layer across the prepared baking sheet, ensuring there is enough space for the air to circulate.
Roast in the oven for 20 to 25 minutes, or until the chicken is cooked through and the vegetables have developed golden, charred edges.
Remove from the oven and immediately drizzle with fresh lemon juice to brighten the smoky flavors before serving.