Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
In a small mixing bowl, whisk together the harissa paste, extra virgin olive oil, and lemon juice until well combined.
Cut the chicken breast into 1-inch bite-sized cubes and place them in a bowl, tossing with half of the harissa mixture.
Dice the red bell pepper, zucchini, and red onion into uniform pieces and spread them out on the prepared baking sheet.
Drizzle the remaining harissa mixture over the vegetables and toss with your hands or a spatula to ensure everything is coated.
Nestle the chicken pieces among the vegetables on the baking sheet, ensuring they are in a single layer for even cooking.
Sprinkle the sea salt and black pepper over the entire tray.
Roast in the center of the oven for 18 to 22 minutes, or until the chicken is cooked through and the vegetables have slightly charred, caramelized edges.
Remove from the oven and garnish with freshly chopped parsley before serving hot.