Add the onion, celery, carrots and garlic to an Instant Pot with the leftover turkey carcass.
Add enough water to hit the 10 cup line in the Instant Pot.
Place the lid on the Instant Pot and set to sealing.
Set the manual timer to high for 60 minutes.
After cooking, allow 20-30 minutes for the pressure cooker to naturally release the pressure, then remove the lid and pour the stock through a fine mesh strainer.
Allow the stock to cool, then transfer to mason jars or plastic containers for storage.
Store in the refrigerator for 3 days or in the freezer for up to 4 months.