Meanwhile, bring another pot with 6 cups cold water to a boil over high heat. Add the mushrooms, bok choy and scallions. Cook just until the mushrooms are tender, about 3 minutes. Add the drained noodles and cook just until the liquid returns to a simmer. Remove the pot from the heat. Transfer about 1 cup of the cooking liquid to a small bowl, add the miso and mix to dissolve the miso. Stir the dissolved miso back into the pot. If you like a stronger flavor, dissolve more miso in some cooking liquid and add it to the pot.
Ladle the soup and noodles into deep bowls and top each with 2 slices of pork. Serve hot, with the red pepper flakes and sesame oil passed on the side so each person can season the soup to taste.
Note: There are two main varieties of bok choy available. Traditional bok choy has long stalks, with wide white bases and dark green, leafy tops, and each head weighs about 1 pound. Baby bok choy is light green and much smaller, and a single head weighs about 7 ounces, which will chop into the 2 cups required for this soup. Unless you don't mind having leftover bok choy (great in stir-fries), look for the baby ones, which can be found at Asian grocers, many natural food supermarkets and more traditional supermarkets, too. If you can't find either bok choy, substitute Nappa cabbage or baby spinach leaves.