Heat the stock over medium heat. Trim two of the lemongrass stalks of their toughest outer layers, then bruise them with the back of a knife; cut them into sections, and add them to the stock with the ginger and about 1/4 of the minced chilies. Simmer for about 15 minutes, longer if you have the time. Peel all the hard layers off the remaining stalk of lemon grass, and mince its tender inner core.
When you’re ready to eat the soup, remove the lemon grass and ginger. Add 1 tablespoon nam pla and the chopped mushrooms. Taste the broth, and add more chilies if you like, as well as some salt if necessary. In the bottom of each of 4 warmed bowls, sprinkle a little each of the chile, lime leaves or zest, lime juice, cilantro and minced lemon grass.
Ladle the soup into the bowls and add a teaspoon of nam pla to each bowl. Serve piping hot.