Roasted Chicken with Celery Root and Red Onion

1 whole chicken (3 1/2 to 4 pounds), patted dry 
Coarse salt and ground pepper 
1 large red onion, cut into 1/2-inch wedges, root end left intact 
1 medium celery root (about 1 pound), peeled and cut into 1/2-inch wedges 
1/4 to 1/2 teaspoon red-pepper flakes 
4 teaspoons lemon zest, plus 4 teaspoons lemon juice

1. Preheat oven to 425 degrees. Season chicken inside and out with salt and pepper. Tuck wing tips underneath chicken and tie legs together with kitchen twine. Place on a rimmed baking sheet and roast 20 minutes. 2. 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.