01 - Whisk together olive oil, zaatar, sumac, lemon zest, lemon juice, garlic, salt, and pepper in a small bowl until well combined.
02 - Gently loosen the skin over the chicken breasts and rub half the marinade underneath. Coat the entire exterior of the chicken with the remaining marinade.
03 - Place lemon slices and onion rings inside the chicken cavity for added moisture and flavor during roasting.
04 - Spread the remaining onion rings, carrot chunks, and lemon slices in a roasting pan. Position the chicken on top, breast side up.
05 - Let the chicken sit at room temperature for 30 minutes before roasting. For deeper flavor, refrigerate covered for up to 8 hours, then bring to room temperature before cooking.
06 - Heat the oven to 400°F.
07 - Place the roasting pan in the middle of the oven. Cook 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 - Allow the chicken to rest for 10 minutes before carving. Serve with roasted vegetables and pan juices.