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.