The trick to caramelizing onions (without leaving them overnight in a slow cooker) is to add a bunch of water to the pan and steam them until they’re completely wilted, then boil off the water and add oil. This will cause most of the onion cells to burst, releasing their water, allowing that water to quickly evaporate so that the oil can do its job raising the temperature of all the onions to the point at which the sugars begin to caramelize.