Peel, core, and roughly chop apples, enough to fill your pot. (Keep in mind that the apples will cook down by about half.) Add apple juice or water (or apple juice concentrate and water), sugar, and cinnamon.
Turn crockpot on high and let the apples cook for about 6 hours until soft. Mash with a potato masher or use an immersion blender to turn them into sauce. Add a splash or two of lemon juice to taste.
Let cool before eating and store in the refrigerator.