Slice the onions and add them to a large bowl.

Melt the butter in the slow cooker (or however)

Add the olive oil, a bit at a time to the bowl of sliced onions along with a bit of salt at a time and mix well by hand.

Continue until all the oil and salt has been mixed into the onions and all the onion pieces are separated. Cover.

Cook on low for 6-8 hours; stir well and replace the lid

If there is a significant amount of water, keep the lid slightly open, allowing the moisture to escape.

Cook for a total of 12-13 hours.

Serve as needed.