Heat the butter in a large skillet set over medium heat. Once the butter has melted, add the onions and cook, stirring occasionally, for 20 minutes, until the onions soften and become golden. Reduce the heat to medium-low, then stir in the onion soup mix. Cook the onions, stirring as needed to prevent burning, until they are dark brown and caramelized, about an additional 30 minutes. If at any point the onions begin to burn, add 2 tablespoons of water to the pan and continue cooking. Set the onions aside.