Optional step: Around the 9-hour mark, place the lid to the slow cooker slightly ajar. Let the lid remain this way for the last few hours of slow cooking to evaporate the excess liquid off the onions.
Refrigerate covered for up to 5 days or freeze for up to 3 months.
This recipe is for plain-Jane basic caramelized onions. You can add in ingredients like a tsp of balsamic vinegar or brown sugar.