Place sauerkraut in a 3-1/2- or 4-quart slow cooker. Trim fat from meat. If necessary, cut roast to fit into the slow cooker.
Sprinkle lightly with salt and pepper. Spread mustard over meat. Place meat on top of the sauerkraut. Add the beer.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
Transfer meat to a cutting board; cool slightly. Slice meat; discard fat.
Serve sauerkraut and meat on Big Marty’s rolls with additional mustard and caraway seeds, if desired. Serving suggestion: side of mashed potatoes (shown here).