Combine beef, egg, bread crumbs, 1 teaspoon Worcestershire sauce and ground mustard in large bowl. Form into four 1-inch-thick oval patties. Press BROWN/SAUTÉ on Crockpot™ Express Pressure Cooker; heat oil on HIGH. Add patties; cook in two batches 4 minutes per side or until lightly browned. Remove to plate. Add onions to Cooking Pot; cook and stir 5 minutes or until softened. Stir in mushrooms; top with patties.
Stir broth, soup, ketchup and remaining 1 teaspoon Worcestershire sauce in medium bowl. Pour mixture over patties. Secure lid. Press MEAT/STEW, set pressure to HIGH and time to 15 minutes. Make sure Steam Release Valve is in the "Seal" (closed) position. Press START/STOP.
Once cooking is complete, natural release pressure 10 minutes. Release remaining pressure. Remove patties to serving plates. Stir cornstarch into water in small bowl; stir into liquid in Cooking Pot. Press BROWN/SAUTÉ; cook 1 minute or until thickened, stirring constantly. Serve over patties; garnish with parsley.