In a 5- or 6-qt. slow cooker, whisk water and beef base; add mushrooms, onion and garlic. Sprinkle roast with pepper; transfer to slow cooker. Drizzle with Worcestershire sauce. Cook roast, covered, on low 6-8 hours or until meat is tender., Remove roast to a serving platter; tent with foil. Strain cooking juices, reserving vegetables. Skim fat from cooking juices. In a large saucepan, melt butter over medium heat. Stir in flour and salt until smooth; gradually whisk in cooking juices. Bring to a boil, stirring constantly; cook and stir 1-2 minutes or until thickened. Stir in cooked vegetables. Serve with roast.