PREHEAT oven to 350°F. Spread potatoes evenly onto bottom of lightly greased 9-inch square baking pan; set aside.
RESERVE 1/4 cup of the steak sauce. Mix remaining 1/4 cup steak sauce with the meat, bread crumbs and onion. Place evenly over potatoes, pressing firmly to form solid layer. Brush with reserved steak sauce.
BAKE 1 hour 10 min. or until meat mixture is cooked through. Sprinkle with cheese; continue baking 5 min. or until cheese is melted. Let stand 10 min. before serving.