Add ingredients to slow cooker, then toss to coat. Don't fill the slow cooker more than 3/4 full.
Set cooker to LOW and cook for 8-10 hours. Stir occasionally, if you are around.
The onions are done when they take on the perfect golden-brown caramelized color, this is a personal preference. If there's too much juice still in the cooker, set the lid ajar and cook another 3-5 hours, but check it regularly to get it where you want it.
When done, package up into various sized containers to go in the freezer, depending on how you plan on using the onions. 2-oz containers are great for adding to appetizers or topping steaks or chicken or sandwiches; 1-cup containers would be easier for a family, and once you open it, you can put it back in the fridge to use over the next week or so. If you plan on making French Onion Soup, it will take this whole batch, probably.
Use a cold water bath to quickly cool your apportioned caramelized onions
Label & Freeze!