Arrange the oxtails in a large Dutch oven. Add the stock, star anise, ginger, onions, garlic, soy sauce, brown sugar, and bean paste, making sure the oxtails are covered in liquid; if necessary, add water to cover. Bring to a boil, then reduce to a simmer and cook, partially covered, for 3 hours, or until the meat is almost falling off the bone. If not tender, continue to cook, checking every 30 minutes. At this point, the oxtails are best if cooled and refrigerated in their liquid for up to 3 days so that the surface fat congeals and the meat develops a rich, savory flavor.
When ready to serve, remove the congealed fat and bring the oxtails to a simmer. Heat for 10 minutes. If the sauce is thin, remove the oxtails and boil the sauce to reduce until it becomes flavorful. Don’t overreduce, or the sauce will become too salty. Discard the star anise and ginger. Return the oxtails to the pot and rewarm while you cook the bok choy.
Bring a pot of salted water to a boil. Add the bok choy and cook for 3 to 4 minutes, or until tender. Drain.
To serve, place 2 bok choy in each of six shallow soup bowls. Ladle over the oxtails and sauce and sprinkle generously with the scallions. Serve steamed rice on the side.
Turn leftovers into chow mein: Remove the meat from the bones and cut into chunks or shred. Cook Chinese egg noodles until just tender. Toss the noodles with the meat, some scallion pieces, and slivers of yellow onion in a little oil in a wok or large skillet, moisten the noodles with just enough braising sauce to create a light sauce, and heat through. Garnish with thinly sliced scallions and serve.