Place the apples in a slow cooker. In a medium bowl, mix the sugar, cinnamon, cloves, nutmeg (if using) and salt. Pour the mixture over the apples in the slow cooker and mix well. You might have to slowly add the apples as they cook down (depending on the size of your slow cooker or crockpot).
It took me nearly 30 minutes to add all the apples.
Cover and cook on high 2 hours.
Turn the slow cooker to low and cook 12 hours, stirring occasionally, until the mixture is thickened and dark brown.
After 12 hours, take the lid off and continue cooking on low another 2 hours.
Stir with a whisk to smooth the apple butter.
To check if the butter is ready, take about a teaspoon of butter in a plate and tilt the plate. If no juice “bleeds” from the butter, it is ready. If it does, cook longer.
Let the butter cool, and then store/can in clean, sterile jars. Cover and store in refrigerate or freeze.