Initial preparation: cut the steak into slices about 1/4" thick by 2‚Äö?Ñ?? long. Place the sliced beef in a large bowl together with half the soy sauce, the sugar, and 1/3 the cornstarch. In another bowl, combine the beef broth, rest of soy sauce and peanut oil. Mix the balance of the cornstarch with the water and hold aside.
Heat 2/3 of the oil (2 tbsp.) in the wok to high temperature. Stir fry the beef until it just loses red color (about 2 1/2 minutes.) Remove beef from wok and add balance of oil, then stir fry diced onion 1 minute. Now add meat broth mixture and stirfry 2 more minutes. Stir in cornstarch/water mixture and continue cooking just until sauce thickens. Serve at once.