01 - In a small bowl, whisk together olive oil, zaatar, sumac, lemon zest, lemon juice, garlic, salt, and pepper to make a marinade.
02 - Gently loosen the skin over the chicken breasts and rub half the marinade under the skin. Rub the remaining marinade all over the outside of the chicken.
03 - Optionally, stuff the chicken cavity with a few lemon slices and some of the onion.
04 - Arrange the remaining onion rings, carrot chunks, and lemon slices in a roasting pan. Place the chicken on top, breast side up.
05 - Let the chicken marinate at room temperature for 30 minutes, or cover and refrigerate for up to 8 hours for deeper flavor. Bring to room temperature before roasting.
06 - Preheat oven to 400°F.
07 - Roast the chicken in the middle of the oven for 1 hour to 1 hour 15 minutes, or until the juices run clear and an instant-read thermometer inserted into the thickest part of the thigh registers 165°F.
08 - Let rest for 10 minutes before carving. Serve with roasted vegetables and pan juices.