Drain the broth through a colander or sieve -retain the liquid!!! - and throw away the contents of the colander. Return the broth to the pan and put it back on low heat.
Add the mushroom soaking water, the mushrooms, and the fish sauce to the pan. Simmer gently.
Bring a large pan of water to the boil. Drain the soaking noodles and add them to the pan of boiling water. Cook them for 30 seconds only. During this 30 seconds, place the serving bowls on the counter., ready to fill with your soup.
Drain the noodles and then divide them between the bowls. Divide the lettuce, coriander, and the spring onions between the bowls, on top of the lettuce. Ladle over the soup and serve at once.
Individuals can add chiles and a squeeze of lime if they like.
Using both a spoon and chop sticks, stir up the soup, mixing all the ingredients together, and slurp it up.