Bring a little bit of olive oil in a sauté pan to medium-low heat, and add the onions. Now comes the waiting - let the onions cook down for about an hour, stirring every 5-10 minutes or so. Be careful to not let the pan get too hot - the onions will burn. Just keep the pan on medium low heat, and let them cook down slowly. It's time consuming, but well worth the wait. For a more comprehensive guide on caramelizing the onion, please see
this article from The Kitchn.