Trim all visible fat from the meat and place it in a slow cooker.
In a medium bowl, combine the onion, water, mushrooms, ketchup, wine, mustard, Worcestershire sauce, salt, pepper and garlic. Pour the mixture over the meat. Cover and cook on low about 8 hours.
Remove the meat from the slow cooker, wrap it in foil and keep warm. Turn slow cooker control to HIGH. In a small bowl, dissolve the cornstarch in water; stir it into the slow cooker. Cover and cook on HIGH for 15 to 20 minutes or until slightly thickened. Slice the beef, and serve the sauce with the meat.
Notes
*If you are preparing this recipe as gluten-free, just be sure to use brands of Worcestershire sauce, Dijon and ketchup that are known to be GF.