Slice the meat across the grain in 1x1/4 inch strips. Sprinkle the beef with cornstarch and soy sauce. Toss to mix, and set aside.
Bring a pot of water to a boil and add the broccoli; return the water to a boil and cook 2 minutes. Drain immediately. Set aside.
Heat the oil in a wok or large non-stick skillet until very hot. Add the onion and gingerroot and stir-fry about 30 seconds.
Add the steak and stir-fry about 2 minutes to brown the edges of the meat. Add the broccoli, sherry, and 2 tablespoons water. Stir constantly until steaming hot, about 1 minute.