Cook the chuck roast or rump roast in the slow cooker with the beef broth, French onion soup, onion, beer, garlic, rosemary (if using), and Worcestershire sauce.
Cook on low for 8-10 hours or on high for 4-6 hours, until the meat is very tender.
Remove the meat from the slow cooker and shred or slice it.
Melt the butter and brush the French rolls or baguette slices with the melted butter.
Serve the shredded or sliced meat on the buttered rolls with the cooking liquid for dipping.
