Put the apples and cinnamon stick, if using, in the slow cooker and pour over
Cover the cooker with the lid and cook on LOW for 8 hours until the apples are very tender. Remove and discard the cinnamon stick, if necessary.
Strain the sauce, reserving the cooking liquid (see Cook’s Tip). Return the apples to the cooker and, using a potato masher or wooden spoon, mash to a texture as chunky as you like. Alternatively, process the sauce through a mouli-legume until smooth, then return to the cooker.
Stir in the sugar and lemon zest, adding a little more sugar and zest, if you like, and stir until the sugar dissolves. Serve hot or chilled.
If not serving immediately, leave the sauce to cool completely, then cover and chill. Keep refrigerated for up to 2 days, or freeze for up to 1 month.