Boil the sake and 150ml of mirin in a saucepan for 4 minutes to evaporate the alcohol.
Add ginger and garlic to the pan and cook for 3 minutes until tender.
Stir in the pepper and miso paste until blended and remove from heat.
Brush steaks with the miso mixture and marinate in the fridge for 3 hours.
Fry shiitake mushrooms in oil until coloured, then add the rest of the mushrooms for 3 minutes.
Deglaze the mushrooms with 100ml of mirin and reduce by half.
Stir in the vinegar and soy sauce before setting the mushrooms aside.
Griddle the onion rings for 7 minutes until caramelised on both sides.
Boil the remaining 250ml of mirin and pour it over the onions in a sealed container.
Cook the steaks on a griddle pan, brushing with extra miso until cooked to your preference.
Rest the beef for 20 minutes before slicing and serving with mushrooms, onions, and wasabi.