Place the rotisserie chicken carcass in the slow cooker. Add onion, carrots, celery, garlic, herbs, bay leaves, and peppercorns.
Pour in enough water to cover everything and fill the pot almost to the top.
Cover and cook on Low for 24-36 hours. I like to set the slow cooker for 12 hours overnight, then check on it in the morning and reset to another 12-24 hours if needed.
Strain through a fine mesh sieve into a large bowl lined with a cheese cloth or paper towel and discard the solids.
Let it cool, then refrigerate. Once chilled, skim off any fat on top if you prefer.
Store in the fridge for up to 5 days or freeze for up to 3 months.
