In a pot, bring the chicken or vegetable broth to a gentle boil.
Add lemongrass, kaffir lime leaves, galangal or ginger, and Thai chilies to the broth. Simmer for 5-7 minutes to infuse the flavors into the broth.
Add the shrimp or chicken and mushrooms to the pot. Cook until the protein is cooked through and the shrimp turns pink or the chicken is no longer pink in the center.
Stir in fish sauce, lime juice, and sugar. Adjust the seasoning according to your taste preferences.
Remove the lemongrass stalks, lime leaves, galangal or ginger slices, and discard them.
Ladle the soup into serving bowls and garnish with cilantro leaves.
If desired, add additional garnishes like sliced red chili, cilantro leaves, and a squeeze of fresh lime juice before serving.
