Blanch the udon noodles according to the package instructions and then shock in cold water. Set aside as you prepare the rest of the dish.
In a small saute pan, combine the soy sauce with the sesame oil, garlic, ginger, green onion, and sugar. Bring to a boil and reduce to a glaze, about 4 minutes. Set the glaze aside until needed.
Combine the shiitake mushrooms and dashi with the water in a 2-quart saucepan and bring to a boil. Reduce to a simmer and cook until the mushrooms are soft and pliable, about 20 minutes.
Strain the broth and return to the pan. Rinse the mushrooms under cold water to cool, and slice thinly. Return the sliced mushrooms to the pan, and set over a low flame to keep warm while you prepare the fish.
Lightly season the fish with salt and white pepper, and brush with the glaze. Wrap in a banana leaf and place in a bamboo steamer. Steam the fish until cooked through, about 5 to 6 minutes.
To serve the dish, divide the noodles among 4 Asian-style soup bowls, and pour 1 cup of the broth in each bowl. Unwrap the banana leaves, being sure to pour any accumulated juices into the soup bowls for added flavor, and place the steamed fish in the bowl with the noodles. Garnish with the green onions and cilantro and serve immediately while hot.