Core and roughly chop the apples, leaving the peels on. Place the apple pieces in the insert of a 5- to 6-quart (4.75-5.5) slow cooker.
Add ¼ cup water, cover, and cook on low for 8 hours or until the apples are very tender.
Transfer the cooked apples to a blender. Puree until very smooth, scraping down the sides as needed.
Let cool, and then serve or store until ready to use. Store in an airtight container in the refrigerator for 3 to 5 days or in the freezer for 4 to 6 months.
