Heat the water with the kelp in a deep saucepan. Just before it boils, remove the kelp and pour in the dried bonito flakes. Boil, stirring, for about three minutes, then strain into a clean saucepan. Add the soy sauce, mirin, salt, and mushroom liquid. Bring to a boil again; taste, adjust seasoning if necessary and cook over medium heat for a few minutes.
Five minutes before serving, heat up the chicken and leeks in the broth. In another pan, cook the noodles according to instructions on the package, then drain and rinse to get rid of the starch.
To serve, place a mound of noodles in each deep soup bowl. Top with one mushroom and separate mounds of chicken, Japanese leeks, and spinach. Gently ladle on the broth and serve immediately. Pass the seven-spice pepper separately.