Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
In a small bowl, whisk together the harissa paste, half of the olive oil, and the lemon juice until well combined.
Coat the chicken breast thoroughly with half of the harissa mixture and let it marinate for at least 10 minutes.
Slice the zucchini into rounds and chop the red bell pepper into bite-sized pieces.
In a large bowl, toss the cauliflower florets, zucchini, and bell peppers with the remaining olive oil, sea salt, and black pepper.
Spread the vegetables in a single layer on the prepared baking sheet and roast for 15 minutes.
While vegetables roast, heat a cast-iron skillet over medium-high heat and sear the chicken breast for 3 minutes per side until a golden crust forms.
Move the vegetables to the sides of the baking sheet, place the seared chicken in the center, and roast for an additional 8-10 minutes until the chicken is cooked through.
Drizzle the remaining harissa sauce over the chicken and garnish the entire tray with freshly chopped parsley before serving.