Build the aromatic base: Warm oil in a pot over medium heat. Add lemongrass, ginger, and garlic.
Cook 2–3 minutes until fragrant, stirring often.
Add broth and simmer: Pour in chicken broth and add kaffir lime leaves if using. Simmer gently for 10 minutes to infuse flavor.
Slip in the chicken: Add sliced chicken and mushrooms (and any sturdy veggies like carrots). Simmer 6–8 minutes, just until chicken is cooked through.
Stir in coconut milk: Reduce heat to low.
Add coconut milk and bring back to a gentle simmer. Do not boil hard to keep the coconut milk smooth.
Season: Add fish sauce, lime juice, and coconut sugar or honey if using. Taste and adjust salt, pepper, and acidity.
Add chilies for heat.
Finish and serve: Remove lemongrass, ginger slices, and lime leaves. Stir in green onions and fresh herbs. Ladle into bowls and serve with extra lime.