Remove sheet from oven and arrange onion and celery root around chicken, turning to coat with drippings. Top vegetables with red-pepper flakes and season with salt and pepper. Roast until vegetables are tender and juices run clear when chicken is pierced between breast and leg (an instant-read thermometer should read 165 degrees when inserted into the thickest part of a thigh, avoiding bone), about 30 minutes, flipping vegetables halfway through. Let chicken rest 10 minutes before carving. To serve, sprinkle chicken and vegetables with lemon zest and juice.