Put the ginger, coriander, mustard seeds, turmeric, and chili powder in a medium saucepan and stir to combine. Cook over medium heat, stirring constantly, for 1 minute.
Add the bok choy, onion, water, and garlic. Cover and cook, stirring occasionally, until the vegetables are tender, 10–15 minutes. Add the walnuts and vinegar and stir to combine. Add the optional salt and season with pe