The noodles here aren't cooked in the soup; they'd absorb too much of the liquid. Instead, they're boiled separately, tossed with sesame oil, and then put into bowls, waiting to be warmed by the hot broth. The bok choy goes into the soup toward the end of cooking, so that a hint of crispness remains. Slideshow: Asian Soups
How to Make It
In a large saucepan, heat the cooking oil over moderately low heat. Add the scallions, garlic, and ginger and cook, stirring occasionally, until fragrant, about 1 minute. Add the soy sauce, red-pepper flakes, water, and broth and bring to a boil.
Add the carrots to the broth and simmer for 5 minutes. Stir in the cabbage and salt and simmer for 5 minutes longer. Add the bok choy and lemon zest and simmer until the bok choy starts to soften, about 5 minutes. Stir in the lemon juice.
Meanwhile, in a large pot of boiling salted water, cook the vermicelli until just done, about 9 minutes. Drain. Return the noodles to the pot and toss with the sesame oil. Put some noodles in each of four bowls and ladle the soup over the noodles.
