Pierce the thigh skin many times with a fork, and rub the meat all over with shio-koji. Wrap tightly in plastic wrap or a plastic bag, making sure to remove all the air.
Marinate for 5-6 hours in the fridge.
After the chicken has chilled for 5-6 hours, cut it into bite-sized pieces.
Cut off the root end of the shimeji mushrooms and shred apart.
Cut the Japanese leek into thin diagonal slices.
Thinly slice the onion.
Mix together the ingredients marked with ＊.
Heat some oil in a frypan, and fry the chicken skin-side down. Cook each side for about 2 minutes or until they have browned nicely. It's okay if the chicken isn't completely cooked through at this point.
Soak up the excess oil with a paper towel, then add the shimeji, onion, and leek to the pan. Pour the mixture of ＊ ingredients into the pan and simmer until the chicken is fully cooked.
Beat the eggs and gently pour into the pan in a circular motion. Cover with a lid and steam until it is half-cooked.
Now it's done. Sprinkle on some ichimi spice or sansho pepper to taste, and dig in!