Preheat your oven to 425°F and line a large rimmed baking sheet with parchment paper.
In a small mixing bowl, whisk together the olive oil, harissa paste, minced garlic, lemon juice, sea salt, and black pepper until a smooth marinade forms.
Slice the chicken breast into 1-inch strips and cut the bell pepper, zucchini, and red onion into bite-sized pieces.
Place the chicken and vegetables into a large bowl, pour the harissa mixture over them, and toss thoroughly to ensure an even coating.
Spread the ingredients in a single layer on the prepared baking sheet, making sure not to overcrowd the pan so the vegetables can roast rather than steam.
Roast for 20 to 22 minutes, or until the chicken is cooked through and the vegetables have developed dark, caramelized edges.
Remove from the oven and allow the chicken to rest for a few minutes before serving to ensure maximum juiciness.