This zippy sauce is intended for dipping pieces of chicken from the pho—a seasoning station between bowl and mouth. "You can drizzle the sauce into the soup if you like, but it will change the flavor of the broth," says Nguyen.
In a small bowl, stir together ginger, lime juice, and sugar until sugar dissolves. Taste and adjust as needed.
Add fish sauce and chile; set aside 15 minutes to meld and develop flavors.