Preheat oven to 160C. Heat a large saucepan or casserole dish over high heat. Add a tablespoon of olive oil. Cook lamb shanks in 2 batches for 1-2 minutes each side or until browned. Remove the lamb shanks and set aside.
Reduce heat to medium. Add a little more olive oil and gently cook the onions for 3-5 minutes until soft. Add garlic, ginger, star anise and cinnamon and cook for 1 minute. Stir in tomato paste, add the lamb shanks, stock, water and tomato to pan. Cover and bring slowly to boil. Bake for 1 hour 20 minutes. Add the sweet potatoes and cook for a further 30-40 minutes or until lamb is very tender. Remove any fat from the surface and season with salt and black pepper. Sprinkle with coriander and serve.