Add enough apple slices to cover the bottom of the slow cooker. Sprinkle 1/2 cup of brown sugar and some of the cinnamon over the slices. Repeat until you’ve used all of the apples, brown sugar, and cinnamon. Add a pinch of salt and pour the apple cider over the apples. Toss the apples with a wooden spoon.
Cover and cook on low for 10 to 12 hours, or overnight. Remove the lid, mash the apples down with a potato masher, and stir to mix. Leave the lid off and continue cooking on low, mashing and stirring occasionally, until the mixture has reduced to about 1/3 and is thick. Turn off the slow cooker and allow the mixture to come to room temperature.
Use an immersion blender (or jar blender) to puree the cooled mixture until it's very smooth. Store covered in the refrigerator for up to 2 months or in the freezer for up to 3 months. You may also process jars using the boiling water method (link in the Notes).
Use 1 cup maple syrup and 1 cup brown sugar to sweeten the apples.
Try this spice mix: 2 teaspoons ground cinnamon, 1/2 teaspoon ground cloves, and 1/2 teaspoon ground allspice.
If using a 4-quart slow cooker, adjust the number of apples (about 8 medium to large apples), and reduce the sugar to 1-1/2 cups.