Season the chicken breast with sea salt and black pepper on both sides.
In a small bowl, whisk together the harissa paste, minced garlic, and half of the lemon juice.
Heat the olive oil in a skillet over medium-high heat and sear the chicken for 5-6 minutes per side until cooked through.
During the last 2 minutes of cooking, brush the harissa mixture onto the chicken, allowing it to glaze and become fragrant.
In a small pot, bring the water to a boil, then stir in the couscous, cover, and remove from heat to steam for 5 minutes.
While the couscous steams, sauté the sliced zucchini in the same skillet used for the chicken until tender-crisp.
Fluff the couscous with a fork and stir in the lemon zest, remaining lemon juice, and chopped fresh parsley.
Slice the chicken and serve it over the fluffy lemon couscous alongside the sautéed zucchini.