1Slice flank steak against the grain in 1/4″ slices. For thicker parts of the flank steak, you may need to slice the strip in half so it’s not too wide.
In a medium bowl, mix beef with soy sauce, rice wine, and sugar. Set aside for 15 minutes.
2While beef is marinating, heat 1 tablespoon oil in a wok or similar on high. Add peppers, mushrooms if using and onions with a sprinkle of salt. Stir and fry for a minute, and remove from heat and set aside. The peppers should still be crisp. After the peppers rest for a few minutes, they will excrete some liquid (amount varies with different peppers). Drain and discard the liquid.
When beef has rested 15 minutes and peppers are ready, mix cornstarch into marinated beef.
3Heat 2 tablespoons oil on high in a wok or similar. Add beef and stir quickly, separating the slices. When beef has started turning color with some pink spots remaining, add vegetarian oyster sauce. Stir quickly to distribute. Add peppers back in. Give a few good tosses and serve with rice immediately.