In a large pot, pour in the chicken or vegetable broth and bring it to a gentle boil over medium heat. Add the smashed lemongrass, torn kaffir lime leaves, smashed bird’s eye chilies, sliced galangal, and minced garlic.
Reduce the heat to a simmer, allowing the aromatic herbs to infuse the broth. Let it cook for 5-7 minutes. Add the sliced mushrooms to the broth and let them cook for about 2 minutes.
Carefully add the shrimp or chicken to the pot, and cook for 3-5 minutes, or until the shrimp turn pink and are fully cooked (or until the chicken is tender).
Add the fish sauce, lime juice, and sugar to the pot. If you want additional heat and depth, stir in 1-2 teaspoons of Thai chili paste (Nam Prik Pao).
Taste the soup and adjust the seasoning with extra lime juice or fish sauce to reach your preferred balance of spicy, sour, and salty flavors. Remove the lemongrass and galangal pieces from the pot to prevent bitterness.
Garnish with freshly chopped cilantro and lime wedges (optional), serve and enjoy!