Slice off the root and top ends of the onions, peel the onions. Cut the onions in half. Lay them cut side down and slice the onions lengthwise to desired thickness.
I use a cast-iron skillet to saute'. Coat the bottom of the skillet with olive oil about (1-teaspoon per onion). Heat the skillet on medium high heat. Add the onion slices and stir to coat the onions with the oil. Spread the onions out evenly over the pan and let cook, stirring occasionally. After about 10 minutes, sprinkle some salt over the onions and or any seasonings such as (fajita or cajun), and if you want to, you can add some sugar to help with the caramelization process.Add a little water to the skillet if they start to dry out. You can use butter instead of the olive oil, or a mixture of the two.
Let cook onions for 30 minutes to an hour or more, stirring every few minutes. As soon as the onions start sticking to the pan, let them stick a little and brown, but then stir them before they burn. After the first 20 to 30 minutes you may want to lower the temperature a little bit, and add a little more oil, if you find the onions begin to start burning. A fork will help you scrape up the browned bits from the bottom of the skillet as they begin to caramelize. As the onions cook down, you may find you need to scrape the skillet every minute or so. Continue to cook and scrape, cook and scrape, until the onions are a rich, browned color.