Combine the diced apples, water, and cinnamon in a large saucepan. Bring to a boil, then reduce heat to low. Simmer for 30–45 minutes, until apples are softened, stirring and mashing occasionally. (I use a potato masher, but if you don't have one, you can use a big wooden spoon and crush the apples against the side of the saucepan.)
Allow the apples to cool slightly, then transfer them to a blender or food processor. Add the spinach and process until completely smooth.
Preheat oven to 175°F and line a large rimmed baking sheet with parchment paper. Pour the apple mixture onto the center of the sheet and use a spatula to evenly spread it over the paper. (If you have an offset spatula, it works well for this!)
Bake the fruit leather for 3 to 5 hours, until it's dry to the touch but still pliable. Sometimes the edges will be dry before the center—if this happens, use kitchen shears to trim off the done parts and return the rest to the oven. It's hard to give an exact time for baking fruit leather, so be sure to check on it every 20–30 minutes so it doesn't overcook!
Once the fruit leather is finished baking, remove it from the oven and let it cool, then use kitchen shears to cut it into strips. (Keep the parchment paper backing—this will keep it from sticking!) Roll the strips and store them in a plastic bag or airtight container.