Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Slice the chicken breast into even 1-inch strips and chop the zucchini, bell pepper, and red onion into bite-sized pieces.
In a large mixing bowl, whisk together the harissa paste, extra virgin olive oil, sea salt, and black pepper until well combined.
Add the chicken and vegetables to the bowl, tossing thoroughly to ensure every piece is coated in the spicy marinade.
Spread the mixture in a single layer across the prepared baking sheet, ensuring the pieces are not overcrowded so they roast rather than steam.
Place the pan in the oven and roast for 20 to 25 minutes, or until the chicken is cooked through and the vegetables have developed caramelized, charred edges.
Remove from the oven and finish with a bright squeeze of fresh lemon juice before serving.