Cut the steak across the grain (diagonally) into very thin slices; place in a
medium bowl, add cornstarch and toss to coat.
Heat the oil in a wok or large skillet until almost smoking; add beef and gingerroot and stir-fry for 2
minutes or until beef is no longer pink on the outside; transfer to a plate.
Add the onion to the wok or skillet and stir-fry until softened, 1 to 2 minutes. Add spinach and stir-fry
until just wilted, about 30 seconds.
In a small bowl, combine broth and ketchup; blend well then add to the pan, along with the browned
beef; stir-fry until the sauce is heated through and coats the beef and vegetables, 2 to 3 minutes;
season to taste with pepper.