A pinch of red-pepper flakes livens up wedges of mellow celery root. Roasting the vegetables alongside the chicken means they cook in the drippings—no oil required.
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.
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.
Lemon Lesson: If you're not using peeled celery root right away, keep it in a bowl of cold water with a squeezed lemon. The acid will prevent the white flesh from browning.