Bring the stock to the boil over medium heat in a saucepan. Add the lemongrass, kaffir lime leaves, galangal, and chiles. Lower the heat to medium-low, cover, and simmer for 15 minutes to let the spices infuse the broth.
Uncover and add the fish sauce, sugar, and mushrooms. Simmer for 5 minutes. Toss in the shrimp and cook for about 8 minutes until they turn pink. Remove from the heat and add the lime juice, green onions, and cilantro. Taste for salt and spices; you should have an equal balance of spicy, salty, and sour. It's a good idea to tell your guest's that the lemongrass and lime leaves are for flavor only and should be avoided when eating the soup.