Vietnamese lamb shanks with sweet potatoes

Name: Vietnamese lamb shanks with sweet potatoes
Ingredients: 2 tbsp groundnut oil; 4 lamb shanks about 350g each; 2 onions halved and cut into half moons; 2 tbsp finely chopped fresh root ginger; 3 garlic cloves finely sliced; 2 red chillies both deseeded, one chopped and one thinly sliced; 1 tbsp soft brown sugar plus 1 tsp; 3 star anise; 2 lemongrass stalks, tough outer leaves removed, then bruised; 1.2l lamb stock; 1 ½ tbsp tomato purée; 4 sweet potatoes peeled and cut into big chunks; 2 tbsp fish sauce; juice 2 limes; big handful mint leaves, torn; handful basil leaves