Whisk the soy sauce and sugar together in a small mixing bowl. Arrange the steaks in a shallow dish and pour over the soy sauce marinade. Rub the marinade into the steaks until evenly covered. Cover and allow to marinate at room temperature for 30 minutes. Pre-heat the oven to 200°C and heat a large, heatproof frying pan over a moderate heat until hot. Drain the steaks from any excess marinade, then coat in the groundnut oil; season with black pepper. Sear in the frying pan for 1-2 minutes on both sides until they are golden brown in colour all over. Transfer to the oven to finish cooking; 5-10 minutes depending on how well done you like your steak. Remove the steaks from the oven, transfer to a plate, cover loosely with aluminium foil and allow to rest for 5 minutes. Meanwhile, melt the butter in a large frying pan over a medium heat and saute the mushrooms with a little seasoning, tossing occasionally, until soft and lightly coloured. Bring a large pan over salted water to the boil and blanch the beansprouts for 30 seconds. Drain and arrange on serving plates. Slice the steak into thin strips and arrange on top of the beansprouts. Spoon the mushrooms around, then sprinkle the peanuts on top of the steak. Serve immediately.