The best way to caramelize your onions is to slow cook the onions over low heat in a large skillet. The key is to make sure your pan and oil are hot before adding in the onion, but you do NOT want them to be too hot, or they'll burn! Once you add in all of your chopped onions, reduce your heat down to medium-low and allow them to simmer for about 20 minutes, stirring occasionally. You want them to start to turn a golden brown, but you do not want that sear on the outside. By cooking them at low heat, this will allow them to caramelize slowly and develop a rich flavor that you'll love in your French onion soup!