Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
In a small mixing bowl, whisk together the harissa paste, half of the extra virgin olive oil, and the lemon juice until well combined.
Place the chicken breast on one side of the prepared baking sheet and brush the harissa mixture evenly over all sides of the meat.
Slice the zucchini into half-moons, chop the red bell pepper into bite-sized pieces, and thinly slice the red onion.
In a separate bowl, toss the zucchini, bell pepper, and onion with the remaining olive oil, sea salt, and black pepper until coated.
Spread the vegetable mixture in a single layer on the other side of the baking sheet, ensuring they aren't overcrowded.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender with slightly charred edges.
Remove from the oven and let the chicken rest for 5 minutes to lock in the juices before slicing and serving alongside the roasted vegetables.