Dissolve 1 tablespoon cornstarch in 3 tablespoons cold water. Combine with the egg in a medium bowl; add the sliced flank steak and marinate in the refrigerator several hours or overnight. Heat the oil in a wok or heavy-bottomed skillet over high heat. Remove the beef from the egg mixture with a slotted spoon; add to the wok and stir-fry until light brown. Add the broccoli and stir-fry until bright green. Remove the broccoli and beef; drain. Remove all but 2 tablespoons of the oil from the wok. Add the green onions and stir-fry 1 minute. Whisk together the sugar, wine, remaining 1-1/2 teaspoon cornstarch, chicken stock, soy sauce, sesame oil, Msg, and pepper. Add the mixture to the green onions and stir briskly. Return the beef and broccoli to the wok. Toss lightly to coat with the sauce and turn out onto a heated platter or bowl. Serve immediately.