01 - Whisk together olive oil, zaatar, sumac, lemon zest, lemon juice, garlic, salt, and pepper in a small bowl until thoroughly combined.
02 - Gently loosen the skin over the chicken breasts. Rub half the marinade under the skin, then coat the entire exterior with the remaining mixture.
03 - Fill the chicken cavity with lemon slices and onion rings if desired for added moisture and flavor.
04 - Spread remaining onion rings, carrot chunks, and lemon slices in a roasting pan. Place chicken on top, breast side up.
05 - Let stand at room temperature for 30 minutes, or refrigerate up to 8 hours. Bring to room temperature before roasting.
06 - Preheat oven to 400°F.
07 - Roast in the center of the oven for 1 hour to 1 hour 15 minutes, until juices run clear and an instant-read thermometer inserted into the thickest part of the thigh reaches 165°F.
08 - Let rest for 10 minutes before carving. Serve with roasted vegetables and pan juices.