Grease the inside of your slow cooker crock with the butter and just leave the excess in the bottom of the crock. Pour in the broth and add the mustard, whisk to combine.
Add the cabbage, mushrooms, and carrot to the slow cooker.
Mix everything together, season with a pinch of black pepper. Cover the slow cooker and set to Low setting and let cook for at least 6 hours, and up to 10 hours. Using the High setting is not recommended or the cabbage will be crunchy.
After I mixed and had the cooker on Low, I realized I had some White Crab mushrooms, so I chopped those and added them.
Mix those in, and now I have this cooking, can never have enough mushrooms.
When ready to serve, taste and season with salt and pepper as desired.
Use a slotted spoon to remove from the slow cooker, serve as a side dish any meal.