In a large pot, bring the lemongrass, lime leaves, galangal or ginger, and water to a boil and then lower the heat to medium. Add the shrimp, tamarind paste, tamarind tendrils (if using), chile peppers, and nam prik pao, and simmer just until the shrimp is cooked through.
Reduce the heat to medium-low. Add the mushrooms, fish sauce, lime juice, and sugar, and stir until the sugar dissolves. Ladle the soup into bowls, garnish with cilantro, and serve. Originally published March 05, 2013