Place roast beef into the insert of your slow cooker. In a 2-cup measuring cup, mix together all remaining ingredients. Pour over roast beef and set the timer for your slow cooker. (5-7 hours on High or 8-10 hours on Low)
Once roast beef has cooked, remove from slow cooker with tongs into a serving dish. Break apart lightly with two forks and then ladle additional gravy/pan sauce over roast beef.
Notes
If you prefer a more pronounced flavor, once the roast beef has been removed, use a fat separator on the gravy and then pour the gravy into a saucepan over medium heat. Simmer until the gravy has reduced by half.