Slice meat into thin slices with the grain. Cut each strip across the grain about 1" long so you have small thin slices. Place in a bowl and add 4 tsp. of soy sauce, beef broth, 1 tsp. cornstarch and black pepper.
In a small bowl, mix 6 tbsp. soy sauce, water and 4 tsp. cornstarch. Set aside.
Heat the wok on high heat. Swirl in 1 tbsp. oil and add the beef, spreading evenly in the wok. Cook undisturbed for 20 seconds letting the beef brown. Using a spatula, stir meat, cooking another 2 minutes. Transfer to a plate.
Add remaining 2 tsp, of oil to wok. Add peppers, water chestnuts, mushrooms and onions and cook about 3 minutes or until tender. Return beef and juices to the wok. Add the sauce and red pepper flakes and stir fry about 30 seconds until slightly thickened.
by The Savvy Kitchen