How to Make It
In a large bowl, season the chicken with salt and black pepper. Add the spring onions, 1/2 cup each of the cilantro and parsley, the preserved lemon, garlic, ginger, paprika, ghee, harissa, cumin, turmeric, cardamom, crushed red pepper, cinnamon and saffron and toss to coat. Cover and let marinate in the refrigerator overnight.
In a large enameled cast-iron casserole, heat 1/4 cup of the olive oil. Scrape the marinade off the chicken and reserve. Working in batches, cook the chicken, skin side down, over moderate heat until browned, about 8 minutes. Add the carrots, fennel, peas, green olives, reserved marinade and 1 cup of the broth. Cover the casserole and bring the liquid to a boil, then reduce the heat to moderately low and cook until the chicken is tender, about 35 minutes. Remove from the heat and let the tagine stand, covered, for about 15 minutes. Add the lemon juice and the remaining 1/2 cup each of cilantro and parsley and season with salt and black pepper.
Meanwhile, put the couscous in a large heatproof bowl. In a medium saucepan, heat the remaining 2 tablespoons of olive oil. Add the yellow onion and cook over moderate heat, stirring occasionally, until lightly browned, about 10 minutes. Add the remaining 2 cups of broth, bring to a simmer and pour over the couscous. Cover the bowl with plastic wrap and let stand for 15 minutes. Fluff the couscous with a fork and season with salt. Serve the couscous with the tagine.