In 4- to 6-quart slow cooker, mix margarine, onion, beef base, salt, pepper and beef.
Cover; cook on Low setting 9 to 10 hours.
About 15 minutes before serving, cook and drain noodles as directed on package. Meanwhile, add tomatoes and rosemary to beef mixture; mix well. Increase heat setting to High; cover and cook 10 minutes longer. Serve beef mixture over noodles.