To make the dashi (Japanese soup stock): Combine the water, kelp, and bonito flakes in a 4-quart saucepan and place over medium-low heat. Allow the water to slowly come to a simmer; this should take about 10 minutes. Turn off the heat immediately just as the stock reaches a boil. Let the stock sit for 1 to 2 minutes and then strain out the solids. Reserve 2 quarts of the dashi to use in the future as a base for soups and stews (it will keep for 1 week in the refrigerator or frozen for several months), and the remaining quart to prepare the miso soup.
Pour the dashi into a wok and place over medium heat. Whisk the miso into the dashi, stirring until smooth. Toss in the mushrooms.
Lightly coat the bottom of a 12-inch bamboo steamer with non-stick cooking spray. Season both sides of the fish fillets with salt and pepper; lay them side by side in the steamer, skin-side up. Cut the ginger lengthwise in strips and put it on top of the fish so the flavor can permeate; put the cilantro on top. Nestle the bok choy in the steamer, side by side, and cover with the bamboo lid. Set the steamer inside the wok, and steam for 15 to 20 minutes until the fish is cooked. Carefully remove the bamboo steamer and add the udon noodles and green onions into the soup. Cook for 1 minute or until the noodles are tender.
To serve: Ladle the miso soup into 2 wide shallow bowls, scoop the noodles into the soup and lay the bok choy and fish on top of that. Garnish with more cilantro and sprinkle lightly with the chili-sesame salt and serve.