Caramelized onions add a scrumptious sweetness to lots of dishes, dips, and sauces, but you may wonder just what caramelizing is. All vegetables and fruits contain natural sugars. When heated, these sugars brown and caramelize, becoming more intense in flavor. Onions are great candidates for caramelizing because they contain plenty of natural sugars. Onions can be caramelized by cooking them slowly in oil. Or they can be caramelized using a little sugar.