This is a simplified version of the traditional Palestinian dish m'sakhan, in which chicken is spiced with sumac and then roasted in the oven over bread. Sumac and za'atar that we love and use so much are combined here with fresh lemon to give the chicken a powerful sharp kick. It works fantastically well and is almost addictive. Try serving the chicken with warm pita bread and a garlicky yogurt sauce, made by mixing Greek yogurt with crushed garlic, olive oil, salt, and pepper.