Start by placing the sliced apples into a saucepan.
Sprinkle the cinnamon evenly over the apples and add sugar if using, then pour in water or apple cider. This will create aromatic steam and a sweet, spicy liquid.
Bring the mixture to a gentle simmer over medium heat, the apples will start to bubble softly as their edges become tender.
Reduce the heat to low and let it simmer slowly, stirring occasionally. Watch as the apples soften and release their juices, thickening the syrup into a fragrant glaze.
Continue simmering until the apples are very tender and the syrup has thickened to your desired consistency—usually about 15 minutes.
Remove the saucepan from heat, and let the cinnamon apple simmer cool slightly. The dish will be spoonable with tender apples immersed in a warm, caramel-colored syrup.