Preheat oven to 400°.
Split carcass in half lengthwise. Place carcass halves in bottom of broiler pan with juices from Roast Garlic-Lemon Chicken.
Bake at 400° for 1 hour, turning bones once after 30 minutes.
Scrape bones, juices, and browned bits into a large Dutch oven. Add peppercorns, 3 quarts water, parsley, if desired, and onion to pan. Bring to a boil over medium-high heat. Reduce heat, and simmer 2 hours; skim surface occasionally, discarding solids.
Strain stock through a sieve into a bowl; discard solids. Cool to room temperature. Cover and chill 8 to 24 hours. Skim fat from surface; discard. Refrigerate stock in an airtight container for up to 5 days, or freeze for up to 3 months.