Directions

• Combine lemon juice and grated garlic in a blender; let stand 15 minutes. Using flat side of a knife, smash black garlic cloves on a cutting board, and scrape to form a paste. Add black garlic paste and salt to blender; puree until mostly smooth and black garlic is very finely chopped, 2 to 3 minutes.
• Place tahini in the bowl of a stand mixer fitted with the whisk attachment; beat on high speed until smooth and creamy, 4 to 5 minutes. Decrease speed to medium, and slowly drizzle in black garlic mixture. (Mixture will thicken but will smooth out again after adding water.) With mixer running, slowly drizzle in 3/4 cup cold water until fully incorporated. Increase speed to high, and beat until mixture is smooth, glossy, and fluffy, about 5 minutes.
• Preheat oven to 450°F, and line a large baking sheet with heavy-duty aluminum foil. Place chicken, backbone side up, on baking sheet. Using kitchen shears, cut along each side of backbone; remove and discard backbone. Flip chicken breast side up, and push down on breast to flatten. Season both sides of chicken evenly with salt. Roast chicken, breast side up, in preheated oven until a thermometer inserted in thickest portion registers 140°F, 40 to 45 minutes, rotating pan after 20 minutes. Meanwhile, make the sumac onions.
• Toss together onion, vinegar, sumac, and salt in a large bowl until onions are well coated. Set aside until ready to serve.
• Remove chicken from oven. (Do not turn oven off.) Tilt pan slightly to collect rendered juices into a baster or large spoon. Baste chicken skin with drippings, and sprinkle with za’atar. Return chicken to oven, and roast at 450°F until a thermometer inserted in thickest portion of chicken registers 155°F, 5 to 10 minutes. Let chicken rest 15 minutes before carving. Serve with sumac onions and black garlic tahini.