Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Chop the cauliflower into small florets, slice the zucchini into half-moons, and cut the red onion into thin wedges.
In a large mixing bowl, whisk together the harissa paste, extra virgin olive oil, lemon juice, sea salt, and black pepper.
Cut the chicken breast into 1-inch bite-sized cubes and add them to the bowl, tossing until every piece is thoroughly coated in the spicy marinade.
Add the cauliflower, zucchini, and red onion to the bowl and toss again until the vegetables are lightly seasoned.
Spread the chicken and vegetable mixture in a single even layer across the prepared baking sheet to ensure even roasting.
Place the sheet in the oven and roast for 20 to 25 minutes, tossing the ingredients halfway through, until the chicken is cooked through and the vegetables are tender with charred edges.
Remove from the oven and garnish with freshly chopped parsley before serving hot.