This recipe courtesy of Jim Botsacos.
Place chopped carcass in stock pot; add enough water to cover bones by 2 inches, about 16 cups. Add onions, carrots, celery, bay leaves, thyme, garlic, peppercorns, and a pinch of salt; bring to a boil. Reduce heat, and simmer, occasionally skimming any foam from surface, for 1 1/2 hours.
Turn off heat; let stock stand for 10 minutes. Strain into another pot, discarding bones and vegetables. Use immediately, or let cool to room temperature before transferring to airtight containers. Stock may be refrigerated for 3 days or frozen for 4 months.