Blend egg white, 2 teaspoons soy sauce, 1 teaspoon cornstarch, ginger, garlic powder, salt and pepper in medium mixing bowl. Add beef and stir to coat. Cover and refrigerate 30 minutes.
Combine cold water, oyster sauce, if using, remaining 3 teaspoons soy sauce, remaining cornstarch, and bouillon granules in small mixing bowl. Combine well and set aside.
Heat 2 tablespoons oil in large skillet. Add beef mixture and stir-fry over medium-high heat until beef is browned. Remove mixture from the skillet and set aside. Heat remaining oil to the skillet and add the mushrooms and onions. Stir-fry over medium-high heat 1 minute. Add bean sprouts and water chestnuts and stir-fry for another minute. Stir in the cornstarch mixture and the beef and cook until thickened and bubbly. Serve with rice and additional soy sauce, if desired.