Rehydrate the kombu and shiitake for 15 minutes in a bowl of water.
Preheat the oven to 200°C.
Place your fish fillet on parchment paper or aluminum foil. Add the kombu and shiitake on top of the fish fillet. Season with soy sauce, sesame oil and chopped chives.
Close the aluminum foil or parchment paper to form a papillote and cook in the oven for 12 to 15 minutes, according to the thickness of the fillet.
Season with lemon juice or ponzu sauce and sansho pepper right before serving.