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 - Place lemon slices and onion rings inside the chicken cavity for added moisture and flavor infusion.
04 - Spread remaining onion rings, carrot chunks, and lemon slices across the roasting pan. Position the chicken breast-side up on top of the vegetables.
05 - Let stand at room temperature for 30 minutes. For enhanced flavor, cover and refrigerate up to 8 hours, then bring to room temperature before roasting.
06 - Heat 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 thigh portion reaches 165°F.
08 - Allow the chicken to rest for 10 minutes before carving. Serve with roasted vegetables and pan juices.