Use a 6-quart slow cooker. Place the bratwurst into the slow cooker and add in the beer, beef broth, mustard, and brown sugar. Stir to combine. Place the onion rings and cabbage on top.
Cover and cook on low for 6 hours, or on high for about 3 hours.
Use tongs to serve the cooked bratwurst on toasted hoagie buns with a scoop of cooked cabbage and onion.