To marinate the chicken, combine the harissa paste, 2 tablespoons of the olive oil, minced garlic, lemon zest and juice, cumin, paprika, salt, and pepper. Add the chicken, coat well, and marinate at least 30 minutes or overnight.
Preheat oven to 425°F. Toss the zucchini, bell peppers, red onion, and cherry tomatoes with the remaining tablespoon of olive oil and a pinch of salt. Spread on a large sheet pan. Nestle chicken skin-side up among the veggies.
Roast for 35–40 minutes, until chicken skin is golden and reaches 165°F, and vegetables are caramelized at the edges.
While the chicken roasts, prepare couscous per package directions, making sure you fluff it up with a fork before serving.
Divide the couscous between bowls, and top each with a chicken thigh and some roasted vegetables. Finish each portion with crumbled feta and chopped parsley.