Infuse the broth. Using the back of a knife, lightly smash the lemongrass on a cutting board, then cut it into 1-inch pieces. Add lemongrass, galangal, kaffir lime leaves, chicken broth, and the white parts of the green onions to a large stockpot. Bring to a boil over high heat, then reduce heat to medium-low and simmer for 10 minutes, stirring occasionally.

Strain. Pour the broth through a fine-mesh strainer and discard the lemongrass, lime leaves, and galangal solids. Return the clear, aromatic broth to the pot.

Build the soup. Add the mushrooms, coconut milk, fish sauce, coconut sugar, and Thai chili paste (if using). Stir to combine. Bring the soup back to a gentle simmer over medium-high heat and cook for 3–5 minutes, until the mushrooms are tender.

Add the chicken and lime. Stir in the shredded chicken and cook for 1–2 minutes, until warmed through. Remove from heat and stir in the fresh lime juice. Taste and adjust — adding more fish sauce (salty), lime juice (sour), sugar (sweet), or chili paste (spicy) as desired.

Serve. Ladle the soup into bowls and garnish generously with cilantro, green onion tops, red chiles, and lime wedges. Serve warm and enjoy!