Shio koji looks unremarkable - a soft, cloudy paste of fermented rice and salt - but it does something to chicken that no marinade of soy and sugar can manage. The enzymes break down the protein overnight, so the meat comes off the grill juicy and tender with a deep savouriness that is hard to place.
There is almost nothing to the recipe. Chicken, koji, a few hours in the fridge, then a hot pan. The one rule worth following is to wipe the excess paste off before cooking, because the sugars in it will scorch long before the chicken is done.
Serve it with rice, shredded cabbage and a small dab of karashi mustard. It is the kind of quiet, everyday Japanese cooking that turns into a weeknight habit.
