Koji is a mould used to inoculate rice, barley and soybeans, kicking off the fermentation that can then be turned into drinks such as sake and shochu, and key pantry ingredients like miso, mirin and soy sauce. It deepens and intensifies flavour and is a key driver of umami.
We buy the koji marinade from Japanese supermarkets. It’s usually labelled “shio koji” in English, and looks very like porridge.
1.2 kg (2.6 lb) Westholme T-bone steak
100g (3.5 oz) koji marinade
3 tbsp olive oil
Cracked black pepper to taste
Rub the steak all over with the koji paste and let it marinate in the fridge for at least an hour (but don’t marinate overnight – the koji will start to cure the meat will affect the meat’s texture, making it soft rather than tender).
Start an outdoor grill using charcoal. You want white, glowing, very hot coals.
Before grilling, rub as much koji off the steak as you can. Koji is high in sugar and burns easily. Brush the steak with the olive oil and season well with salt.
Grill the steak on one side until a deep golden brown crust forms (about 5 or 10 minutes depending on size of steak) then turn it over and cook the other side for another 5 or 10 minutes. I like it cooked rare to medium-rare, and aim for an internal temperature of 52C / 125F. Before I take the steak off the grill, I sometimes brush a little of the marinade back onto it and caramelise it quickly over the coals – not more than a minute each side.
Rest the steak in a warm spot (in a low oven or on a rack above the grill) for 10 minutes, then slice it and serve it dressed with some more olive oil and black pepper.