Red chili pepper (also chile or chilli)
(such as Thai bird; ends trimmed; smashed with a knife)
(fresh; chopped leaves and tender stems)
Combine stock, galangal, lemongrass, and lime leaf in a medium saucepan. Bring to a simmer over medium heat, and then reduce heat to maintain a simmer for 10 minutes. Discard the galangal and lemongrass.
Add the coconut milk and increase heat to medium. Add chicken, mushrooms, galangal matchsticks, and Kaffir lime leaves, and cook until the chicken is completely cooked and the mushrooms are tender, about 5 minutes.
Add the fish sauce, lime juice, and red chiles. Remove from heat. Divide the soup between four bowls, making sure each has a red chile. Garnish with cilantro. Adjust flavor with more lime juice and fish sauce as necessary.
Galangal and kaffir lime leaves can be found at Southeast Asian supermarkets.
If unavailable, ginger can be substituted for galangal, and 4 strips of lime zest can be substituted for lime leaves.