Peel and thinly slice all of the onions then cut into half moons. 

Place the onions in to the slow cooker so that it is roughly three quarters full.

Add the olive oil and salt to the slow cooker ensuring all of the onions are coated. 

Set your slow cooker to the lowest setting, cover the slow cooker with the lid and cook for up to 10 hours on LOW. 

If you're around while the onions are cooking, stir them occasionally — this will help them cook more evenly, but don't panic if you are asleep / at work! 

After 10 hours, the onions should be be golden-brown and soft. There will probably be a lot of liquid which you can drain should you choose. 

For a more jam like texture, cook for another 2 to 3 hours on low with the lid askew. (Check the onions frequently during this time and stop cooking when you are happy with the consistency).