Trim all visible fat from the beef roast.

Place the roast in a slow cooker.

In a small bowl, combine minced onion, water, crumbled shiitake mushrooms (rinsed), ketchup, red wine, Dijon mustard, Worcestershire sauce, salt, pepper, and crushed garlic.

Pour the mixture over the beef roast in the slow cooker.

Cover the slow cooker and cook on LOW for approximately 8 hours.

Remove the cooked beef roast from the slow cooker and slice it.

Keep the sliced meat warm.

Turn the slow cooker to HIGH heat.

In a separate small bowl, dissolve cornstarch in water to create a slurry.

Stir the cornstarch slurry into the liquid in the slow cooker.

Cover the slow cooker and cook on HIGH for 15-20 minutes, or until the sauce has thickened to your desired consistency.

Serve the thickened sauce over the sliced beef roast.