Add all ingredients to a slow cooker. Cook on low for 10 hours (I do it overnight).
If desired, cook for another 3-5 hours with the lid ajar to evaporate the extra liquid and thicken the onions. This will also result in a darker, sweeter onion. If you’re in a rush, drain the extra liquid from the onions.
Store covered in fridge for 4-5 days or freeze for up to 4 months.