How to Make It
In a large wok, heat 2 tablespoons of the peanut oil. Add the onion and ginger and stir-fry over high heat until golden, about 5 minutes. Scrape into a mortar and add the black beans and ground chiles. Mash until coarsely ground. Stir in the vinegar.
Heat the remaining 2 tablespoons of peanut oil in the wok. Add the black bean mixture and stir-fry over moderate heat until fragrant, about 1 minute. Add the mussels, cover and steam until they open, about 4 minutes; discard any mussels that don't open. Transfer the mussels and sauce to a large bowl, sprinkle with the sliced scallion and serve hot.