Place chicken bones or the leftover chicken carcass into a large stock pot.
Add onion, carrots, celery, garlic, parsley, and thyme to the pot.
Pour in the filtered water, making sure the ingredients are fully submerged.
Add the apple cider vinegar and let everything sit for 10 to 15 minutes before heating.
Bring the pot to a gentle boil over medium heat.
Reduce heat to low and allow the broth to simmer uncovered.
Skim off any foam or impurities that rise to the surface during the first hour.
Continue simmering on low heat for 18 to 24 hours to extract maximum nutrients.
Remove from heat and allow the broth to cool slightly.
Strain the broth through a fine mesh strainer into a large bowl.
Discard solids and season with salt if desired.
Let cool completely before transferring to storage containers.