For the beef with miso, heat the sake and mirin together in a small saucepan for 3-4 minutes, or until the alcohol has evaporated. Add the ginger and garlic and cook for 2-3 minutes, or until tender. Add the pepper and miso paste and stir until blended. Remove from the heat.
Brush the steaks lightly with the mixture. If possible, marinade, covered, in the fridge for 2-3 hours, but bring back to room temperature before cooking.
For the pickled Japanese mushroom salad, heat the oil in a frying pan and fry the shiitake mushrooms until coloured. Add the remaining mushrooms and fry for 2-3 minutes.
Deglaze the pan by pouring in 100ml/3½fl oz mirin, then bubble until reduced by half. Pour in the vinegar and soy sauce, then stir, remove from the heat and set aside.
Preheat a griddle pan to hot. Cook the onion rings for 5-7 minutes, turning halfway, until caramelised on both sides. Transfer to a heatproof lidded container and set aside.
Bring the remaining 250ml/9fl oz mirin to the boil, then pour it over the onions. Seal the container and leave to cool.
For the beef with miso, preheat a heavy-based griddle pan over a medium heat. Cook the steaks until a crust has formed. Continue cooking and turning, brushing the steaks with more black-pepper miso each time they are turned, until cooked to your taste. Be careful not to let them catch, or the miso will go bitter. Leave to cool for at least 20 minutes.
Slice the steaks and arrange on a platter. Serve with the mushrooms, onions and wasabi.