This is the best Tom Yum Soup recipe. To maximize the citrus flavor of the fresh lemongrass, crush the base and stem with the flat side of a chef's knife before slicing or before adding it to a recipe. If using larger pieces of lemongrass, remove them before serving.
1In a mortar, combine the garlic, chilies, cilantro stems and pepper, and grind together with a pestle until a thick paste forms, adding 1 Tbs. water if needed to facilitate grinding. Alternatively, combine all the ingredients in a mini food processor and process to a paste. Set the chili paste aside.
2Remove and discard the stems from the mushrooms. Thinly slice the caps. Trim and crush the lemongrass stalks. Smash the galangal slices with the side of a chef's knife.
3In a stockpot over high heat, warm the oil. Add the mushrooms and sauté until they begin to brown, about 2 minutes. Stir in the lemongrass, galangal slices and shallots, and sauté until fragrant, about 2 minutes more.
4Pour in the stock, add the lime zest and bring to a boil. Reduce the heat to medium, stir in the chili paste and simmer for 3 to 4 minutes. Add the fish sauce, lime juice and palm sugar and stir to mix well. Simmer for 5 minutes to allow the flavors to develop.
5Just before serving, stir in the shrimp. As soon as they begin to turn color and are opaque, after about 3 minutes, remove the soup from the heat. Ladle the soup into individual bowls, garnish with the shredded basil and cilantro, and serve immediately.