Remove excess fat from meat. Cut meat into 1-inch pieces. Place onions in a 3 1/2-or 4-quart slow cooker. Top with mushrooms and meat. Combine in a medium bowl soup, tomato paste, Worcestershire sauce, dry mustard, and pepper. Pour mixture in cooker.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours. Serve over hot cooked noodles (optional).