Special equipment: a slow cooker
1. Set a large skillet over medium heat and add the butter. Once melted, add the onions and cook until starting to soften, 2 to 3 minutes. Add the green chillies and chipotle peppers and cook for a minute.
2. Add the cheese and let it melt. Add the enchilada sauce and give it all a stir. Transfer to a slow cooker to keep warm.