Reduce the heat to medium low, cover, and cook for an additional 30-40 minutes, stirring frequently, or until the onions take on a rich brown color. Note:
Keep an eye on the amount of liquid in your onions, since they will release liquid as they cook. If your pot lid does not have a steam vent, you may need to uncover your onions on and off to allow some of the moisture to evaporate. Covering it will help keep the onions from burning, but you don't want them to stew in their own juices. Also:
Take care not to overcook the onions - this step requires a lot of patience. While turning the heat up will make them brown faster, they will be sautéed rather than caramelized. Caramelization is essential for the onions to release their sugars and get that delicious sweet flavor.