Cut the chicken breast into equal pieces. Mince the onion and chop the garlic clove finely. Cut the half preserved lemon into small pieces, or zest the skin if using a regular lemon. Coarsely chop the cilantro.
Heat the tablespoon of olive oil in a braising pan or tagine over medium-high heat. Brown the chicken pieces on all sides until lightly colored. Add the minced onion and garlic and cook briefly until the onion is translucent. Sprinkle the teaspoon of ras el hanout over and cook for another minute.
Pour the chicken broth into the pan and immediately add the lemon pieces and green olives. Turn the heat low, cover the pan and let everything simmer gently for twenty minutes. Steam the cauliflower couscous until cooked in the last four minutes.
Divide the steamed cauliflower couscous evenly among the plates. Spoon the warm chicken tagine with the olives and sauce over the couscous. Finish the dish by sprinkling the chopped cilantro over it.