1.Bring a medium pot of water to a boil and quickly blanch the shanks for 1-2 minutes. Discard water and rinse the shanks.
2.In the insert of the slow cooker, place the shank pieces in one layer at the bottom of the pot. Add green onions, ginger, star anise, soy sauce, rice wine, black vinegar, soy sauce, brown sugar, and enough water so that the shanks are covered.
3.Cover and cook on low for 5-6 hours, or until the meat is very tender and soft.
4.Remove the shank pieces and let cool overnight in the refrigerator.
5.Thinly slice and serve. You can drizzle sesame oil on top plus cilantro.