Preheat oven to 400°. Combine 2 tbsp. coriander, 2 tsp. sumac, the cumin, pepper, and 1 tsp. salt. Rub chicken with 1 1/2 tbsp. oil, then inside and out with spice mixture. Put lemon inside cavity. Set chicken, breast up, on a rack in a roasting pan. Tie legs together loosely with kitchen twine.
Roast chicken 45 minutes, then baste every 15 minutes or so with pan drippings until a leg joint wiggles easily and juices run clear, about 30 minutes more.
Heat remaining 1 1/2 tbsp. oil and the butter in a frying pan over medium heat. Add raisins, currants, 1 1/2 tsp. sumac, and remaining 1 tsp. coriander and 1/2 tsp. salt. Cook, stirring, until puffy and sizzling, 2 to 3 minutes. Stir in a splash of water, the chickpeas, and parsley and cook until hot. Stir in hazelnuts.
Spread rice on a platter. Arrange chickpea mixture over rice, then set chicken on top and remove twine. Serve with yogurt, the roasted lemon, and more sumac to add to taste.
*Find in the spice aisle or at worldspice.com
Note: Nutritional analysis is per serving without rice.