Use a 4-quart slow cooker. Put the meat into the bottom of the slow cooker insert. In a small bowl, combine the paprika, cumin, coffee, brown sugar, salt, and garlic. Rub this paste on all sides of the meat. Separate the onion rings and place on top of the meat. Pour in the beer.
Cover and cook on low for 8-10 hours, or until meat is fork tender. Serve on top of a bed of mashed potatoes.