In a small bowl, combine the coffee, brown sugar, kosher salt, black pepper and fresh rosemary.
Rub this mixture onto the beef brisket and place in the slow cooker on top of the sliced onions.
Pour the beef broth, stout beer, balsamic vinegar and Worcestershire sauce into the slow cooker.
Slow cook on high for 5-6 hours, or on low for 10-12 hours.
Remove the brisket from the slow cooker, place on a cutting board and shred the meat using two forks, then place the shredded meat back into the sauce in the slow cooker.