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 - Optionally place a few lemon slices and some onion rings inside the chicken cavity for added moisture and flavor.
04 - Arrange remaining onion rings, carrot chunks, and lemon slices in a roasting pan. Position the chicken on top, breast side up.
05 - Let the chicken stand at room temperature for 30 minutes, or cover and refrigerate for up to 8 hours. Bring to room temperature before roasting if refrigerated.
06 - Set oven to 400°F.
07 - Roast in the middle 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 registers 165°F.
08 - Allow the chicken to rest for 10 minutes before carving. Serve with roasted vegetables and pan juices.