Combine the beef, mushrooms, onions, tomato paste, broth, wine and 3 tablespoons soy sauce in the bowl of a slow cooker. Cover and cook on HIGH for 4 hours or on LOW for 6 hours, or until beef is tender.
In a small bowl, stir together the sour cream, remaining 1 tablespoon soy sauce, cornstarch and black pepper; whisk into the hot meat mixture and cook an additional 30 minutes or until sauce has thickened slightly.
Serve over pasta or rice and garnish with parsley, if using.
