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 using your fingers. Rub half the marinade beneath the skin, distributing evenly over the meat.
03 - Rub the remaining marinade over the entire exterior surface of the chicken, ensuring complete coverage.
04 - Place lemon slices and onion rings inside the chicken cavity for added moisture and flavor during roasting.
05 - Arrange remaining onion rings, carrot chunks, and lemon slices in the bottom of a roasting pan. Position the chicken on top, breast side up.
06 - Let the chicken stand at room temperature for 30 minutes before cooking. For deeper flavor, cover and refrigerate for up to 8 hours, then bring to room temperature before roasting.
07 - Preheat oven to 400°F.
08 - Place the roasting pan in the center of the oven. Roast 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.
09 - Remove the chicken from the oven and let rest for 10 minutes to allow juices to redistribute. Carve and serve with roasted vegetables and pan juices.