In a large pot over medium heat, heat oil. Add ginger and cook until fragrant, 1 minute, then add mushrooms and cook until soft, about 6 minutes.
Add broth, coconut milk, and fish sauce and bring to a boil. Add chicken, reduce heat, and simmer until chicken is no longer pink, about 15 minutes.
Turn off heat and stir in lime juice.
Garnish with cilantro and chili oil (if using) before serving.
