Place the stew meat into the bottom of a 6-quart slow cooker.
Season the beef with the black pepper and garlic powder.
Scatter the sliced onions and mushrooms evenly over the beef.
To the slow cooker add the beef broth, brown gravy mix, dry Italian dressing mix, Worcestershire sauce, and steak sauce and stir until combined.
Cover and cook on LOW for 7–8 hours or HIGH for 4–5 hours, until the beef is fork tender.
Once the beef is tender, whisk together the cornstarch and cold water in a small bowl until smooth.
Stir the cornstarch slurry into the slow cooker.
Cover and cook on HIGH for an additional 20–30 minutes, until the gravy has thickened.
Gently stir the beef, onions, and mushrooms into the thickened gravy.
Sprinkle with chopped parsley before serving.
Serve over mashed potatoes, seasoned rice, or buttered egg noodles.