In a 4-6 quart slow cooker, combine butter, onion, beef base, salt, pepper and beef; mix well.
Cover; cook on low for 9-10 hours.
About 15 minutes before serving, cook noodles as directed on package. Add tomatoes and rosemary to beef and mix well. Increase heat to high; cover and cook an additonal 10 minutes. Serve beef over noodles.