Mix first10 ingredients. Place onion, green pepper and celery in a 5-qt. slow cooker; place roast over top. Pour tomato mixture over roast. Cook, covered, on low until meat is tender, 5-6 hours., Remove roast. Strain cooking juices, reserving vegetables. Transfer juices to a small saucepan; skim fat. Mix cornstarch and water until smooth; stir into cooking juices. Bring to a boil; cook and stir until thickened, 1-2 minutes. Serve roast and vegetables with gravy. Freeze option: Place sliced beef and vegetables in freezer containers; top with gravy. Cool and freeze. To use, partially thaw in refrigerator overnight. Heat through slowly in a covered saucepan, stirring gently and adding a little broth or water if necessary.