
Trim off the tough ends of the baby bok choy. Separate the larger outer leaves, then halve the cores lengthwise. Soak the bok choy in a large bowl of cold water for 5 minutes. Gently rub the stems to remove any dirt trapped between the leaves. Rinse well and drain.
Heat the sesame oil in a medium pot over medium-low heat. Add the zha cai, ginger, and the white parts of the green onion. Cook, stirring occasionally, until fragrant and the ginger just begins to brown, 3 to 5 minutes.
Pour in the chicken stock and bring to a boil. Reduce the heat to a simmer and cook for 3 minutes to infuse the broth.
Add the bok choy and return the soup to a gentle simmer. Cook until the bok choy is tender but still crisp, 1 to 2 minutes.
Add the green parts of the green onion. Taste the soup and adjust the seasoning with more chicken bouillon or mushroom powder if needed. If you'd like a brighter, tangier flavor, stir in a splash of the zha cai pickling liquid. Drizzle with a little more sesame oil, if desired. Serve hot.