Remove beef and vegetables from slow cooker; cover to keep warm. Pour juices from slow cooker into 1-quart saucepan; heat to boiling. Mix cornstarch and cold water until smooth; stir into juices. Cook, stirring constantly, until sauce has thickened. Serve sauce with beef and vegetables. Note: This recipe was tested in slow cookers with heating elements in the side and bottom of the cooker, not in cookers that stand only on a heated base. For slow cookers with just a heated base, follow the manufacturer’s directions for layering ingredients and choosing a temperature.