Cut the steak in really thin slices across the grain. Combine with peanut oil, cornstarch and soy sauce. Set aside and let marinate for at least 15 minutes.
In the meantime, prepare the rice noodles according the package directions.
Thinly slice the shiitakes, grate the ginger and mince the garlic. Cut the scallions into 5 cm/2-inch pieces.
Combine the ingredients for the sauce in a small bowl.
Heat 3 tbsp peanut oil in a wok over high heat until smoking-hot and oil is shimmering. Add the beef, cook undisturbed until the beef begins to sear, about 30 seconds. Then stir-fry for 1-2 minutes until the beef is lightly browned but not cooked through. Transfer the beef and any juices in the wok to a plate.
Again, heat 3 tbsp peanut oil over high heat, add shiitakes. Stir-fry for about 1-2 minutes or until golden. Add garlic and ginger, stir for 1 minute.
Return the beef to the wok, add the sauce and the scallions, stir-fry for 1 minute more. Add the rice noddles and stir to coat evenly. Serve!
