Preheat your oven to 400°F (200°C) and lightly grease a large roasting pan or cast-iron skillet with oil. Pat the whole chicken completely dry inside and out with paper towels to ensure a crispy skin.

In a small bowl, whisk together the harissa paste, minced garlic, ground cumin, ground coriander, olive oil, lemon zest, salt, and black pepper until it forms a smooth, fragrant paste.

Carefully loosen the skin over the chicken breast and thighs, then rub half of the harissa mixture directly onto the meat under the skin. Spread the remaining marinade evenly over the entire outside of the chicken skin.

Stuff the chicken cavity with the lemon halves and onion wedges to infuse moisture and flavor from the inside out. Tie the chicken legs together with kitchen twine and tuck the wing tips behind the back.

Place the chicken in the prepared roasting pan and roast in the preheated oven for 70 to 75 minutes. The skin should be deeply browned and caramelized, and an instant-read thermometer inserted into the thickest part of the thigh should register 165°F (74°C).

Remove the chicken from the oven and let it rest on a cutting board for 10 to 15 minutes before carving to allow the juices to redistribute. Carve into pieces and serve hot, drizzled with the flavorful pan juices alongside fluffy couscous.