In a 5- to 6-quart slow cooker combine cabbage, bratwurst, potatoes, sweet pepper, and onion. In a large bowl whisk together broth, mustard, vinegar, salt, black pepper, and celery seed. Pour over all in cooker.
Cover and cook on low-heat setting for 6 to 7 hours or on high-heat setting for 3 to 3-1/2 hours. If desired, top each serving with cheese.