In small bowl, combine fish sauce, miso, and soy sauce. Stir into a paste and set aside.
In large sauce pan, heat sesame oil. When hot, add leeks, ginger, shallots, and bok choy. Stir until lightly golden. Add chicken stock to pan. Bring to a simmer and add mushroom and seaweed. Continue to simmer until mushrooms are cooked. Mix miso soy sauce paste into the broth. Continue to simmer.
Cook udon noodles according to the package. When fully cooked, add to the ingredients in the sauce pan.
This soup can be served with a soft boiled egg, green onion, and Sriracha sauce (if you want a kick).