Peel and thinly slice all of the onions into half-moons. Transfer all the onions to the slow cooker — the slow cooker should be half to three-quarters full.
Drizzle the olive oil or melted butter and the salt, if using, over the top of the onions. Toss to evenly coat all the onions with a thin glaze of oil.
Cover the slow cooker and cook for 10 hours on LOW. If you're around while the onions are cooking, stir them occasionally — this will help them cook more evenly, but isn't strictly necessary. After 10 hours, the onions will be golden-brown and soft, and they will have released a lot of liquid. If you like them as they are now, stop cooking and pack them up.
Cook an additional 3 to 5 hours with the lid ajar (optional): If you'd like jammier, more concentrated onions with a deeper colour, continue cooking for another 3 to 5 hours on LOW. Leave the lid ajar so the liquid can evaporate. Check every hour and stop cooking whenever the onions look and taste good to you.
Refrigerate or freeze the onions: Remove the onions with a slotted spoon and transfer them to refrigerator or freezer containers. If any liquid remains in the slow cooker, transfer the liquid to a separate container — this can be used as cooking broth in another recipe. Onions will keep in the refrigerator for one week or in the freezer for at least 3 months.
It's almost impossible to overcook these onions, but I've found that 10 hours makes soft, lightly browned onions that still have a bit of bite. These are perfect for adding to soups and topping sandwiches. For jammier, even more deeply caramelized onions, continue cooking for another few hours, but this time leave the lid ajar so the liquid can evaporate. (Any liquid remaining after cooking is fantastic added your next soup or risotto, by the way.) Given the über-long cooking time, I've found that it's easiest to let the onions cook overnight. Choose a night when you'll be around in the morning in case you want to cook the onions an extra few hours. If you're planning to freeze a portion of your batch (and you should!), I recommend freezing them in containers of multiple sizes: ice cubes of caramelized onions are great for last-minute burger and sandwich toppings, cup-sized portions can be used for pizza and pasta toppings, and larger containers are perfect to throw directly in a soup.