Preheat your oven to 425°F (220°C) and line a large sheet pan with parchment paper for easy cleanup.
Cut the zucchini into half-moons, the red bell pepper into strips, and the red onion into wedges.
In a small mixing bowl, whisk together the harissa paste, extra virgin olive oil, lemon juice, and minced garlic until well combined.
Place the chicken breast and all the chopped vegetables onto the prepared baking sheet.
Pour the harissa mixture over the chicken and vegetables, then toss thoroughly with your hands or tongs to ensure everything is evenly coated.
Spread the ingredients out into a single layer and season the entire tray with sea salt and black pepper.
Roast in the oven for 20 to 25 minutes, or until the chicken is cooked through and the vegetables have developed a slight char.
Remove from the oven and let the chicken rest for 5 minutes before slicing it into strips.
Serve the sliced chicken over the roasted vegetables and drizzle with any remaining juices from the pan.