Caramelizing onions cooks them to a rich brown color and an intensified sweetness. This process takes time to dissolve the strong sulfur compounds, so don’t try to rush it or you’ll end up with browned onions instead of richly flavored caramelized ones that are as sweet as sugar. To caramelize onions, cut the onions into thin slices. Heat 2 tablespoons olive oil or butter per onion in a large skillet over medium heat. Add the onions, and cook 25 to 30 minutes, stirring often, or until deep golden brown. Sweet onions take longer to caramelize than other varieties due to their higher moisture content.