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