01 - Set the oven temperature to 425°F.
02 - Pat the chicken dry with paper towels. Rub olive oil thoroughly over the entire chicken, including under the skin where accessible. Season liberally with kosher salt and black pepper inside and out.
03 - Insert lemon halves, rosemary sprigs, smashed garlic cloves, and quartered onion into the chicken cavity.
04 - If using, distribute the carrots, celery, and baby potatoes evenly around the chicken in the roasting pan.
05 - Tie the chicken legs together with kitchen twine and tuck the wing tips underneath the body to ensure even cooking.
06 - Place the chicken on the center oven rack and roast for 1 hour and 15 minutes, or until juices run clear and an instant-read thermometer inserted into the thickest thigh registers 165°F.
07 - Allow the chicken to rest for 10 minutes before carving. Serve alongside the roasted vegetables and pan juices.