Rub beef roast with garlic salt and black pepper. Place roast in the bottom of a slow cooker.
Mix together Worcestershire, soy sauce, and red wine vinegar in a small bowl; pour over roast. Sprinkle onion soup mix over roast. Pour water into the slow cooker around roast, not over the top.
Cover and cook on Low until no longer pink in the center and meat is fork tender, 4 to 6 hours.
Transfer sauce from the slow cooker into a saucepan. Warm over medium heat until bubbling, then mix in cornstarch to thicken.