Peel and thinly slice all of the onions into half moons. Transfer all the onions to the slow cooker. Ideally, the slow cooker should be 1/2 to 3/4 full.
Drizzle with olive oil and toss the onions to coat them with the oil.
Sprinkle with salt.
Cover and cook on LOW for about 10 hours. If possible, you can stir them occasionally, but this is totally optional. If you like deeper colored onions, cook 2 to 4 hours more with the lid ajar to let the excess moisture escape. Check them every hour or so until they have reached they have the color and taste that seems good to you.
Remove the onions with a slotted spoon and store in covered containers in the refrigerator or freezer.