Mix Moroccan spice ( grind it first in a spice grinder if it contains whole spices) with Greek yogurt, reserve 2 tbsp of the mixture and set aside. Smother chicken with prepared marinade, cover and refrigerate for 2 hours or overnight.
Preheat the oven to 400F/200C. Place thickly sliced vegetables and 2 cloves of garlic not-peeled in a big roasting pan, don't overcrowd or the vegetables will steam. Sprinkle with lemon zest, salt, 1 remaining clove of crushed garlic, 1 tbsp of olive oil and roast for 30 minutes.
Take the chicken out of the marinade and grill on a barbecue or a griddle for 8-10 minutes on one side and 5 minutes on the other, reduce or increase time depending on the thickness.
Put couscous in a medium bowl, bring chicken stock to a boil and pour over couscous, cover and let it absorb the liquid for 8-10 minutes, then fluff it with a fork.
When the vegetables are done remove them from the oven, cover and keep warm.
Peel the roasted garlic and put it in a food processor with the reserved marinade, mayonnaise and water, blend until smooth, add more water if needed to a desired consistency.
Place sliced chicken breast on top of couscous along with roasted vegetables and serve with additional sauce and chopped cilantro.