For the beef:
Heat a pan over a high heat and add the groundnut oil. Place the steaks in the pan, and cook until they have browned on 1 side. Then, turn over to cook the second sides. Cook according to the desired degree of doneness: 1 to 2 minutes for rare, 2 to 3 minutes for medium and even longer for well done.
When the steaks are finished, pour in the the remaining marinade, and cook for 30 seconds until the sauce is sticky. Lift the beef out of the pan, and transfer the portions to warmed serving plates. Cover with foil to keep warm.
Add the shiitake mushrooms to the pan, and cook until softened. You can also add a splash of water to help create some steam to cook the mushrooms.