Place the chicken carcass and any reserved bones (wings are wonderful for broth) in a medium pot. Add the onion, celery, carrot, and any additional desired aromatics. Cover with about 8 cups of cold water. Bring to a simmer and cook for 45 minutes to an hour. Allow to cool. With a slotted spoon or spider, remove the large chunks of vegetables and bones from the pot. Strain the broth through a sieve into a container and store in the refrigerator for immediate use or the freezer for future use.