Preheat oven to 425 F.
Place quartered chicken pieces in a large mixing bowl with 2 tbsp oil, 1 tsp cumin, 1 1/2 tsp sumac, cinnamon, allspice and 1 tsp salt and black pepper. Mix well to combine, then spread out on the baking sheet.
Roast until chicken is cooked through, approximately 35-45 minutes. Remove from the oven and set aside. Keep remaining juices in the pan.
Meanwhile, put 2 tbsp oil into a large saute pan and place over medium heat. Add pine nuts and cook for 2-3 minutes, stirring until the nuts are golden brown and smell nutty. Transfer to the prepared bowl and set aside.
Add the remaining 1/4 cup oil to the pan along with onions and 3/4 tsp salt. Return to medium heat for 15 minutes, stirring every few minutes, until the onions are soft. Add 2 tbsp of sumac, the remaining 2 tsp cumin and black pepper to taste. Mix with onions until fully coated then remove from heat and set aside.
Assembly: preheat the broiler and slice flatbread into quarters. Place under the broiler for 2 minutes, to crisp up and then arrange on the edges of serving platter.
Top the flatbread with half the caramelized onions, followed by roast chicken pieces and drizzle with any remaining juices in the pan. Spoon the remaining onions over the top and sprinkle with pine nuts, parsley and remaining 1 1/2 tsp of sumac.
Serve with yogurt and lemon wedges.