With sharp knife, score both sides of meat in diamond pattern, at 1-inch intervals, making cuts 1/8-inch deep.
Whisk water, ketchup, vinegar,oil, Worcestershire sauce, mustard and pepper in medium bowl. Pour into plastic zip-lock style bag. Add meat; turn to coat well. Refrigerate overnight.
Prepare outdoor grill with hot coals on one side of grill, or heat one side of gas grill to hot.
Remove meat from marinade; pat dry. Reserve marinade.
Sear meat over direct heat 5 minutes on each side. Move meat to side of grill not over heat; grill, covered, 20 minutes or until meat thermometer registers 145° F for medium-rare. Let stand 10 minutes before slicing on the diagonal.
To make sauce, boil reserved marinade in small saucepan 5 minutes. Stir in cornstarch mixed with 2 tsp water. Lower heat and simmer 2 minutes until thickened. Serve with steak.
Servings: 6