In a large bowl combine 4 minced cloves garlic, 3 tablespoons olive oil, 2 tablespoons sumac, 1 tablespoon salt, 2 teaspoons cumin, 1½ teaspoons allspice, and 1½ teaspoons coriander. Mix into a paste.
Trim excess skin from 3 pound bone-in, skin on chicken thighs and drumsticks.
Using a knife, slash each piece of chicken all over (4 or 5 times); add chicken pieces and 1 very thinly sliced red onion to spice mixture. Toss to coat chicken with spices, working some of spice mixture and onion slices under skin of chicken. Cover and chill for 1–4 hours.
When ready to cook, preheat oven to 375ºF with racks set in upper and middle position. Arrange ¼ cup pine nuts on a small baking sheet; toast in oven until golden brown and fragrant, 6–8 minutes. Set pine nuts aside. Increase oven to 425ºF.
Arrange chicken in an even layer on a quarter sheet pan; sprinkle onions over top. Roast chicken on middle rack until juices run clear and an instant-read thermometer inserted in the thickest areas, but not touching the bone, registers 172ºF, about 30 minutes.
Turn oven to broil and move chicken to upper rack, broil until skin is browned, onions start to char, and an instant-read thermometer inserted in the thickest areas, but not touching the bone, registers between 175ºF and 180ºF, 5–15 minutes more (all broilers are different—keep an eye on your chicken! This could take just 5 minutes for you while other oven can take up to 15 minutes).
Remove chicken from oven and let rest. Turn oven off and place breads directly on middle rack—heat just to warm through, about 4 minutes.
Transfer breads to a platter, nestle chicken and onions alongside bread; drizzle with some of the chicken juices. Sprinkle pine nuts and parsley over top. Serve with lemon wedges and thinned tahini (see notes).