In the center of a 6-inch square piece of cheesecloth, place the lemongrass, ginger, chilies and lime leaves. Draw up the sides to form a pouch and tie with kitchen twine.
In a large pot, combine the stock, mushrooms, fish sauce, and sachet in a large pot. Bring to a boil. Reduce the heat and simmer for 5 minutes. Add the shrimp, coconut milk, and cilantro, and simmer until the shrimp are cooked, 3 to 4 minutes.
Remove from the heat and discard the sachet. Season, to taste, with salt or additional fish sauce. Serve with jasmine rice.
Yield: 4 to 6 servings