Green Tahini Sauce
Pulse garlic, parsley, tahini, lemon juice, and ½ cup water in a food processor, adding more water if needed, until smooth (sauce should be the consistency of a thin mayonnaise); season with salt
Cover and chill.
Chicken & Assembly
Preheat oven to 400°.
Toss the chicken, onions, garlic, lemon, sumac, allspice, cinnamon, broth, and ¼ cup oil in a large resealable plastic bag; season with salt and pepper.
Let marinate at least 2 hours.
Place chicken, onions, garlic, and lemon on a rimmed baking sheet, spooning any remaining marinade over and around the chicken.
Sprinkle with za’atar and roast until the chicken is browned and cooked through (~45–55 minutes)
Meanwhile, melt butter in a small skillet over medium-high heat.
Add pine nuts and cook, stirring often, until butter foams, then browns, and nuts are golden brown (be careful not to burn), about 4 minutes; season with salt.
Slice chicken breasts, if desired.
Serve chicken with roasted onion and lemon, topped with pine nuts, with green tahini sauce and lavash.