Heat the oil in a 12-inch cast iron skillet over medium heat. Add the onions and sauté for 5 minutes, or until starting to soften.

Add the salt, reduce the heat to medium-low and cook for 50 to 80 minutes, stirring every few minutes, or until the onions are very soft, golden brown, and caramelized. The timing will depend on the size of your onions and the heat of your stove.

If at any point the onions start to stick to the bottom of the pan, reduce the heat to low. If the onions aren’t deeply brown after 50 minutes and you’d like to speed things up, you can turn the heat higher to get more caramelization. Stir continuously to prevent burning.