Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Chop the cauliflower, zucchini, and red onion into uniform, bite-sized pieces for even roasting.
In a small bowl, whisk together the olive oil, harissa paste, lemon juice, sea salt, and black pepper.
Place the vegetables in a large mixing bowl, drizzle with half of the harissa mixture, and toss until well coated.
Spread the vegetables in a single layer on the prepared baking sheet and roast for 20-25 minutes until tender and slightly charred.
While the vegetables are roasting, coat the chicken breast thoroughly with the remaining harissa mixture.
Heat a large non-stick skillet over medium-high heat and sear the chicken for 6-7 minutes per side until the internal temperature reaches 165°F.
Allow the chicken to rest for 5 minutes before slicing it into strips.
Serve the sliced chicken over the bed of roasted vegetables and garnish with freshly chopped parsley.