Bring chicken stock to a boil in a saucepan; add lemon grass, galangal, cilantro, green onion, and kaffir lime leaves. Stir, reduce heat, and simmer for 5 minutes.
Mix chicken, fish sauce, sugar, and red pepper flakes into stock mixture; stir and simmer for 5 minutes. Add coconut milk, shiitake mushrooms, and cremini mushrooms; stir and simmer for 5 minutes.
