2–3 Thai chilies, crushed (adjust to your heat preference)
1/4 cup fish sauce
1/4 cup fresh lime juice
1 tablespoon sugar (or to taste)
1 cup straw mushrooms, halved (or white button mushrooms)
1/2 cup cilantro, roughly chopped, for garnish
1 tablespoon chili paste in oil (Nam Prik Pao) (optional, for color and depth)
Instructions
Prepare the aromatics: Lightly bruise the lemongrass stalks with the back of a knife. Tear the kaffir lime leaves. Slice the galangal thinly.
Bring the broth to a boil in a large pot over medium-high heat.
Add the bruised lemongrass, sliced galangal, and torn kaffir lime leaves to the boiling broth. Reduce heat and simmer for 10 minutes to infuse the broth with flavor.
Add the crushed Thai chilies and the mushrooms to the pot. Simmer for 3 minutes until the mushrooms soften slightly.
Stir in the fish sauce, lime juice, and sugar. Taste the broth and adjust seasoning—you want a strong balance of sour, salty, and sweet.
Add the shrimp to the simmering broth. Cook for 2-3 minutes until the shrimp turn pink and opaque. Do not overcook.
If you want a richer, slightly colored broth, stir in the chili paste in oil now.
Remove the pot from the heat. Discard the lemongrass stalks and galangal slices before serving, or instruct diners to eat around them.
Ladle the soup into bowls, ensuring each serving gets shrimp and mushrooms. Garnish generously with fresh cilantro.
Notes
For a creamy Tom Yum (Tom Yum Nam Khon), stir in 1/4 cup of evaporated milk or coconut milk just before serving.
You can substitute chicken breast, sliced thinly, for shrimp. Add the chicken before the mushrooms and cook until done before adding the final seasonings.
If you cannot find fresh galangal, use ginger, but reduce the amount by half as the flavor is different.
For a stronger flavor base, simmer the aromatics for 15 minutes before adding other ingredients.