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
• 1½ 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