Spray 5- to 6-quart slow cooker with cooking spray. Add beef (if necessary, cut beef in half to fit in cooker). Reserve 1/2 cup cooking sauce; refrigerate while beef is cooking. In medium bowl, mix remaining cooking sauce and bouillon granules with whisk; pour over beef. Sprinkle with onion.
Cover and cook on Low heat setting 7 to 8 hours or until beef is tender. Remove beef from slow cooker; set aside and keep warm. In small bowl, mix reserved cooking sauce and flour until smooth. Gradually stir flour mixture into sauce in slow cooker. Increase heat setting to High; cover and cook about 15 minutes, stirring occasionally, until sauce has thickened. Cut beef into slices. Serve beef with sauce and mashed potatoes.