In a 5- or 6-qt. slow cooker, whisk the water and beef base together. Then add mushrooms, onion and garlic.
Sprinkle the roast with pepper and place it in the slow cooker. Then drizzle Worcestershire sauce over the meat.
Cook the roast, covered, on low for 6 to 8 hours or until the meat is tender. When it's done, remove the meat from the slow cooker. Set the roast on a serving platter and tent it with foil.
Strain and skim the fat from the cooking juices, setting the vegetables aside. In a large saucepan, melt butter over medium heat. Stir in flour and salt until smooth, then gradually whisk in the cooking juices. Bring the contents of the pot to a boil, stirring constantly, and cook for 1 or 2 minutes or until thickened. Add the cooked vegetables back in before pouring the gravy over the roast.
