Place the chicken carcass, onion, garlic, celery, and leeks (if using) into a large pot, and cover with water.
Bring to a simmer and cook for 3 hours, skimming off any fat debris that rises to the surface. Remove from the heat and strain the broth. Season, if desired, and cool completely.
Skim the stock after chilling. Removing excess fat is easier once the stock is chilled since it rises to the top.
Storage: Refrigerate in airtight containers for up to a week or freeze for future use.