01 - Whisk together olive oil, zaatar, sumac, lemon zest, lemon juice, minced garlic, kosher salt, and black pepper in a small bowl until thoroughly combined.
02 - Gently lift the skin over the chicken breast area and rub half of the marinade evenly underneath the skin directly onto the meat.
03 - Rub the remaining marinade over the entire surface of the chicken, ensuring even coverage on all sides.
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 roasting pan to create a vegetable bed. Position the chicken on top, breast side facing up.
06 - Let stand at room temperature for 30 minutes before cooking. For deeper flavor penetration, cover and refrigerate up to 8 hours, then bring to room temperature before roasting.
07 - Heat oven to 400°F.
08 - 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 part of the thigh reaches 165°F.
09 - Remove from oven and let rest for 10 minutes to allow juices to redistribute throughout the meat.
10 - Carve the chicken and serve alongside roasted vegetables with pan juices spooned over the top.