Coat inside of Crockpot™ slow cooker with nonstick cooking spray. Layer onion on bottom of Crockpot™ slow cooker.
Combine beef, egg, bread crumbs, 1 teaspoon Worcestershire sauce and ground mustard in large bowl. Form into four 1-inch-thick oval patties. Heat large nonstick skillet over medium-high heat. Add patties; cook 2 minutes per side or until lightly browned. Remove to Crockpot™ slow cooker. Stir soup, water, ketchup and remaining 1 teaspoon Worcestershire sauce in medium bowl. Pour mixture over patties; top with mushrooms. Cover; cook on LOW 3 to 31/2 hours. Garnish with parsley. Serve with potatoes and peas, if desired.