12 (2- to 2 1/2-inch-thick) oxtail pieces (about 4 1/2 pounds), fat trimmed
2 cups low-salt chicken broth
1 large onion, halved, thinly sliced
1/2 cup soy sauce
8 large garlic cloves, peeled
8 whole star anise*
6 (1/4-inch-thick) rounds fresh ginger
3 tablespoons dark brown sugar
1 tablespoon Chinese brown bean sauce (not ground)**
12 to 18 yu choy, baby choy sum, or baby bok choy
Chopped green onions
*Brown star-shaped seedpods; available in the spice section of some supermarkets and at specialty foods stores and Asian markets.
**Available at Asian markets.