Toss the onions with salt and evoo until well coated.

Cook in slow cooker on low for 10 hours. The onions are ready at this point, but there will likely be a lot of liquid. Use the liquid for soups or stews or remove the lid and cook for another 4 hours until all the liquid evaporates. 

Store the caramelized onions in an airtight container in the fridge for about 2 weeks or in the freezer for 3 months.