Carmelization is the browning of sugar. It is a widely used cooking term in reference to browning the natural sugars in foods through the application of heat during the cooking process. Carmelization adds flavor and in the case of onions, practically transforms them. During the carmelization process the onions become sweet tasting and soft enough to melt in your mouth. To do it well, carmelizing onions happens slowly; very slowly using the method below.