In a large skillet, melt 2 tablespoons of butter and add the sliced onions. Cook while stirring occasionally for 2 minutes, then add a tablespoon of water and continue cooking and stirring while spreading the onions evenly on the pan. Keep adding water whenever the onions dry out and start sticking to the pan. Cook until the onions are soft and caramelized (reached a rich brown color). The process takes about 15 minutes. Remove the onions from the pan and set them aside.