- About 14 cups water
- 6 ounces dried rice-stick noodles
- 1 tablespoon peanut oil or canola oil
- 1 teaspoon finely grated fresh ginger
- 2 stalks lemongrass—top third discarded, tough outer leaves trimmed,
smashed lightly and cut into 2-inch lengths
- 1 pound sweet potatoes, peeled and cut into 1/2-inch dice
- 5 tablespoons soy sauce
- Heaping 1/4 teaspoon crushed red pepper
- 1 small head napa cabbage, sliced crosswise 1/3 inch thick (5 packed cups)
- 1 cup unsweetened coconut milk
- Juice of 1 lime, plus lime wedges for serving
- 2 1/2 cups small cilantro leaves and sprigs
Bring 8 cups of water to a boil. Remove from the heat, add the rice noodles and let soak until the noodles are softened, about 5 minutes. Drain and transfer to a large bowl.
Heat the oil in a large saucepan. Add the ginger and lemongrass and cook over moderately low heat, stirring, until fragrant, about 2 minutes. Add the remaining 6 cups of water, cover and bring to a boil over high heat. Add the sweet potatoes, soy sauce and crushed pepper and season with salt. Lower the heat, cover and simmer until the potatoes are tender, 7 to 10 minutes.
Add the cabbage and coconut milk and simmer, uncovered, for 5 minutes. Stir in the lime juice and cilantro. Add the noodles, ladle the soup into bowls and serve with the lime wedges.