In a medium saucepan over high heat, combine the broth, lemongrass, and ginger and bring to a boil.
Reduce heat and simmer for 15 minutes. Discard the ginger.
Add the noodles and mushrooms and cook until tender, 2 to 3 minutes. Stir in the scallions.
Divide the noodles and broth among bowls. Top with the bean sprouts, basil, and chili oil, if desired.