Scallion-Ginger Broth

In a large saucepan, heat oil over medium-high. Add scallion whites, ginger, and garlic; cook, stirring, until scallions begin to soften, about 3 minutes. Add broth and fish sauce. Bring to a boil; reduce to a simmer and cook until flavors are blended, about 5 minutes. Add stir-ins (if using) and simmer until desired doneness. Add lime juice and garnish with scallion greens if desired.