Preheat your oven to 425°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Cut the chicken breast into 1-inch bite-sized cubes and place them into a large mixing bowl.
Prepare the vegetables by slicing the red bell pepper and red onion into strips, and breaking the cauliflower into small, uniform florets.
In a small bowl, whisk together the harissa paste, extra virgin olive oil, sea salt, and black pepper until well combined.
Add the vegetables to the bowl with the chicken and pour the harissa marinade over everything, tossing thoroughly to ensure an even coating.
Spread the chicken and vegetable mixture in a single, even layer onto the prepared baking sheet.
Roast in the oven for 20 to 25 minutes, or until the chicken is cooked through and the vegetables have developed a slight smoky char on the edges.
Remove from the oven, drizzle with fresh lemon juice, and garnish with chopped fresh parsley before serving.