Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Cut the chicken breast into uniform 1-inch cubes and place them into a large mixing bowl.
Prepare the vegetables by chopping the cauliflower into bite-sized florets, slicing the zucchini into half-moons, and dicing the red bell pepper.
In a small ramekin, whisk together the harissa paste, extra virgin olive oil, sea salt, and black pepper until well combined.
Pour the harissa oil mixture over the chicken and vegetables in the mixing bowl, tossing thoroughly with your hands or a spatula to ensure every piece is deeply coated.
Spread the mixture out in a single, even layer on the prepared baking sheet, making sure not to overcrowd the pan so the vegetables roast rather than steam.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the cauliflower edges are golden and charred.
Remove from the oven, immediately drizzle with fresh lemon juice, and garnish with finely chopped fresh parsley before serving.